The last thing any of us want is a job done wrong due to lack of communication. In a perfect world, the team is on the same page and works out issues at the data stage. Unfortunately, about 20% of our jobs go this way. The industry average is about 5%. My team gets all parties talking at the front of a job due to experience and repeatedly work with firms who understand we are there to make things go as smoothly as possible.
Engineering firms are not to blame. Their reason for not being cooperative may be they are too busy to have a nice long talk about what you found wrong with their job. With proper coordination and your experience as a data engineer, doors will open and there will be better, quicker interaction with designers. Here is how to work things out.
We need a starting point to get things rolling. In our case, it is the building of the model provided after the contractor has won the job. If everything is scheduled correctly, the data is received before field work is ready to start, and there is time for building and review. While building the model, we are looking for a lot of different things.
- Overall quality of the plans.
- With experience, you can tell when things were thoughtfully prepared or just kicked out the door.
- Obvious issues with elevations, COGO, and drainage.
- A few spot grades that are not correct is normal. We will fix them and report it to the contractor.
- The COGO, (Coordinate Geometry) is another thing. When the layout of a site is not consistent from page to page, we need to let people know. A building must fit, cars need to park, and water needs to be managed. If the site permit calculations call for 165 parking spots and the grading sheet only shows 140, we need to alert the team.
- Laws regarding the handling of onsite storm water have changed and become more restrictive. Most sites need to contain and properly drain runoff. Know the rules and verify the cubic footage of water as compared to the plans. If there is a difference, everybody will thank you for not letting the wheels fall off that wagon.
- Experience with the engineer and contractor. When working with known entities, communication is easier. We know who to email and where to get results. With new people in the mix, it is best to get acquainted quickly and establish your main contacts.
- Share the model with everyone. At the start of the project, try and get the right person with the contractor and engineer in the email loop.
When you have a feel for how good things are and who is committed to making things happen, you are now on course to get things taken care of.
Dealing with Issues
There will be things that need to be changed for the project to work, at least on the screen. Sometimes we see an issue that does not look right, and we bring it to the attention of the contractor and engineer only to find out they wanted it that way. Usually, it needs to be changed. How you present the issue is more than half the battle.
- Never bring up a problem without offering a solution. There are two reasons for this:
- First, everybody will know you are familiar with the project.
- Second, nobody wants to figure things out. We are the professionals in making a surface, we should have some clue for success.
- Put together a well thought out email to the group.
- This means that when they read the email, they should have no questions.
- Do not send out something that says, “call me.” Yes, there are times on complex projects that some type of discussion is important, but for the other 95% of the time, be clear and complete in this communication.
- Always put some deadline on the request but don’t go around wanting everything in an hour.
- Pick your battles and leave as much time as possible but be sure to put a limit on it. If you do not get results, they should know you will go ahead and change things or just leave that section out of the model. That will get you answers.
- Don’t sweat the small stuff.
- A fat-fingered elevation is no cause for a Zoom call. Just make sure you note the changes made in a communication to the core group.
- Everybody is busy. The engineers’ process for dealing with the problem you present may take a meeting on their part.
- The message here is to note issues early on so the rest of the project can move along while the details are worked out.
When working on a project, we want everybody to get a copy of our data. It is always better to have several eyes on your work so there are no surprises down the road. In over 80% percent of our jobs, we will provide working files to the surveyors to help them with their portion of the job. This covers several factors:
- The surveyor can look at the data and let us know if they feel changes are in order.
- We are already producing a lot of the information needed for layout, so we can add points or a subgrade file for the survey crews to get to work.
- The survey bill to the contractor will be reduced. No surveyor wants to be stuck in the office doing calculations to send to the field.
It has taken a lot of years to get to the point where we have the trust of most of the large engineering and surveying firms in the country. Now that they are familiar with our work and competence, we are welcomed into a job. If this is your first time with a group, proceed slowly and earn their trust to prove your capable of providing accurate information.
Take Off Professionals is always open to sharing our experience and knowledge. Contact us online for assistance with dirt takeoffs, 3D models and more, or call us today at (623) 323-8441.
One of my superpowers, (maybe my only one) is the ability to find and know how to use obscure commands. I have an advantage in that I am constantly helping engineers on various projects. They may want to streamline a process or be stumped on how to get through an issue. Having a variety of projects to work on keeps me in front of new commands as well as old favorites.
You never really get to know all the commands in a program, few of us DO everything a software program is capable of. An issue presents itself when you’re required to step outside your daily routine. This can involve a process you barely learned about in training, requiring you to do something difficult, just to forget it until you need to perform the process again.
How do you find a command you never use and do just enough to get the process done and back to work? I will go through how I locate and proficiently use those seldom used commands.
For this offering, I will be looking through Carlson Software on AutoCAD Civil 3D. The program is mature and there are a lot of commands that are deep in menus or even only accessible by command line. Trimble Business Center, being a bit newer, isn’t as complex. The fact that it is module based will limit your access to commands outside your normal routine. The challenge comes when you take on new responsibilities, add modules and start hopping back and forth.
How many times have you encountered an issue that could be solved with a command you used two years ago? You forget the name and don’t really want to go through the entire command list to find it. I suggest this approach. When you need to find a command, take a step back and put yourself in learning mode. If you try and grab and go with a command, you may not get it right and the next time you need it, the process begins again.
To make sure this never happens, do some work to make things easier the next time. There is a way in Carlson to make points at the intersections of lines. I will dig until I find it but to save time, I will make a “command sheet.” I’ve listed the items I put on the sheet below. They can all go into a Word document so you can search command words to make things easier. As I go through the process for getting familiar with a command, you can choose the elements you want to put into your version of a command sheet.
- Command Name
- Name the software uses for the command.
- Add keywords to describe what it does and how to apply it.
- Prior and possible uses for the command.
- Tip: When you are in learning mode, watch how others are using the command.
- Command Location
- Both menu and command line descriptions.
- A walk through of how the command works. Add screen shots as well as dialog to make it clear. Remember, you will not look at this for months, in addition, it will be shared with others in the office, and it needs to be self-explanatory. I sure do not want to field a call about a command I do not know well.
For this example, I will drill down into a Carlson command.
Command Name: Create Points from Entities
Keywords: Points, create points, points from lines, intersections, radius points, extra points
Location: Survey Menu> COGO> Create Points from Entities. Command Line: AUTOPNTS
With these basics I can go back and search my document to locate the command for future use. At this point, I want to work with the command to get familiar with it so I can do some documentation.
Dialog Box Notes
- Verify symbol and layer for draw points.
- Always separate attribute layers to keep screen cleaner.
- Check points elevated by the surface.
- Always locate on Z axis.
- Do automatic point numbering, try, and use alpha-numeric for clarification in the model.
- Point order can be left to right or start at the top or bottom.
- Description settings if needed.
With these parameters listed and noted, you will save time the next time you use the command and get things looking the way you want. Here is the dialog box with the desired information and appropriate settings. These are 2D points so they will not be located in 3D. Once the dialog box is populated the way you want, you can move to the next.
The dialog box can get you into trouble. Any time you are using a command, be sure to save the project first before shutting down the program. The reason is it can cause issues like you may get points where you do not need them as well as way too many points. The points in Carlson go to a coordinate file and would need to be removed even if you close or undo the command. Proceed with care.
Consider accessing the help files as they can actually have useful information. Here is part of the Help instructions for this command.
This command will create Carlson points on selected entities. The points are stored in the current coordinate (.CRD) file and drawn on the screen. For arcs and polylines with arc segments, points are created at the radius points of the arcs as well as the PC and PT.
In the first options dialog, there are settings for the point attributes. To have points obtain their elevation from the selected entities, unselect the Prompt for Elevations toggle and select the Locate on Real Z Axis toggle. After you have specified the point options, a secondary dialog appears which allows you to specify the entity types to process. The Point Order setting controls the sequence of the new point numbers to be in the order of selection or in a direction like left to right. Under the Description Settings, Prompt for Description At Each Point will prompt you at the command line for a description for each individual point. Prompt Per Entity will ask you for a description per each highlighted entity. Use Entity Layer for Description will assign the layer name to the description. When Entity Layer for Description is checked, the layer name of the entity will be used as the description for the created point. Same Description For All Points will prompt you for a single description for all points. The Use Text as Description applies to points from Text entities where the description is set to the text string. The Use Block Name as Description applies to points from Insert entities and the program sets the point description to the block name.
The second options dialog has processing settings. When Avoid Duplicates with Existing Pts is checked, this routine will not create a point if a point with the same coordinates already exists in the current coordinate (.CRD) file. The Draw New Points option creates point entities in the drawing. Otherwise, the new points are only stored to the coordinate file. The Draw Existing Matched Points option applies to the Avoid Duplicates option for the case when a duplicate is found in the coordinate file and not yet drawn.
That is a lot of information that will come in handy. I will also usually copy and paste the help dialog into my command sheet if nothing else to have more searchable words. I also save the file as a pdf and use Bluebeam to search; it does a great job and notes where the text is as well as words around it for context.
Other Considerations for Seldom Used Commands
- This is to be shared with others in your group. I suggest creating a document intended to be shared by everyone.
- Do not use this as a substitute for training. This is more of an advanced search tool with written reminders of a command you learned how to use some time ago. I see too many mistakes when a command is executed, and problems arise when something goes wrong, and the operator does not understand.
- Many of these commands can be used for another purpose than intended. This Create Points From Entities command can be used to create layout points for staking curbs and buildings. Most people use it to densify a model or elevate point cloud data with local feature lines now correctly elevated.
- The keyword section can contain random entries that can bring the command back to memory. For example, I have an entry “command C Lawson showed me at dinner.” When the file gets over 100 commands, you sometimes need to stretch the box.
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Creating an exact earthwork estimate is an essential first step to many takeoffs. You want a bid that accurately reflects the cost of labor and materials so you can get the project fee you deserve and minimize financial risks. At Take-off Professionals, our experienced team of engineers creates precise earthwork estimates based on your designs.
This post will explain what earthwork estimates are and the three methods we can use to create them.
Earthworks — Cutting and Filling
Earthwork is the engineering process of moving, removing or adding soil, rock or other materials to change a specific location’s topography. To create a 3D earthwork calculation, compare the location’s current topography with the contractor’s proposed design. The process of creating an accurate earthwork estimation consists of calculating how much material the contractor will need to add — or fill — and take away — or cut.
Cutting is removing material from the site, and filling refers to adding material. Both are crucial for creating the topography reflected in the contractor’s design. Engineers will calculate the cut and fill quantities to reach a mass balance and use that mass balance to provide an accurate earthwork estimate.
The Three Estimating Methods
There are three main estimating methods that engineers can use when creating 3D data for earthwork in construction. These methods calculate the dirt and material quantities required while providing mass haul analysis for the construction site. However, some are more accurate than others.
1. The Average Method
We typically use the average method for smaller projects that require us to ascertain the levels at all grid points. Because this method is the simplest of the three, we can only use it when a project requires either filling or cutting. The average method provides inaccurate estimates if you use it for projects that use both cutting and filling.
2. The Block Method
Our team uses the block method — also called the division of square method — to determine volume for medium-sized projects that require leveling. The block method is more straightforward than the section method and more accurate than the average method. You can expect some margin of error when using the block method on projects involving both filling and cutting, but much less than the average method.
3. The Section Method
The section method is the most complex and precise way to calculate an earthwork estimate. We use this technique when working on large-scale projects, such as dams, railway systems and roads. Although the section method requires many steps and more complicated calculations, it’s the best method to ensure a precise estimate that will save you money.
Get Accurate Estimates With Take-off Professionals
Our team of full-time engineers is on hand to provide you with precise earthwork estimates and essential 3D data. Contact us to learn more about our products or request a quote for our earthwork services.
We have all been there. Get a set of plans, open them, take a look, and wonder what possessed the team responsible to perform the way they did. I started my career in the offices of a developer/builder. I got to see firsthand how the process goes, as well as the changes that must happen. The journey often ends up with something completely different than originally planned.
I wrote this article for many of the younger entrants into the civil world to help shed light on the process. This may make weird site plans a little more understandable.
There are two types of builder groups; public and private. For this discussion we will look at private work. Private work can be further broken down into commercial and local. This article covers commercial builders.
In the civil construction world, commercial development relates to projects done by a large company in different geographic areas. Think of a big box store or restaurant.
Here are some general specifications:
- They have centralized operations and often work on sites they may never visit.
- Their staff does a lot of projects. Operations are streamlined and efficient. When they get busy, questions can sometimes take too long to get answered.
- They have little emotional investment in the process.
- Deadlines and opening dates are the only thing that matters. I have seen corporate firms do full redesigns to placate local building officials without blinking an eye.
- They are not afraid to spend money to get things done. The fast-track construction concept came about just for big box retail.
- They are not used to dealing in your area. Try not to suggest, “we do it this way around here.”
- Junior people handle stuff first. Get used to clearly explaining an issue so the next person up the ladder has all pertinent data.
The biggest factor in commercial work is the time from land acquisition to opening day. Pay and bonuses for employees involved are measured by expediency of their work. A month delay for a big retailer translated into hundreds of thousands of dollars in lost revenue. They still have a bottom line. Quick decisions to problems are important to keep in mind.
Over 15 years ago, I wrote an article for a magazine that included the thought, “all the good dirt is gone.” The nice flat building sites with well compacted soil and no garbage were all developed and all that is left is the stuff that is expensive and difficult to work with. The land is acquired by the company and the process begins. For this part, both commercial and private work follow the same course.
The selection of an engineering firm takes one of two roads. The company has a nationally registered firm, meaning they have people with a license in each state they work in. The other track is to contact a local firm that has experience in the area to get things going. I have not seen much difference in the choice. An advantage of a local firm is that if you get the job, communication could be smoother.
Next, a survey of the property is performed and submitted to the engineer and owner. These will be ALTA Surveys. This means they conform to the American Land Title Association guidelines that follow strict standards for real estate transactions. I remember a job involving a McDonald’s restaurant in Las Vegas that had three surveys performed to verify the plot. The land was priced by the square foot, so every little bit made a difference for all involved.
The survey is just 2D and clearly outlines the property boundary. At the same time, a topography map is usually made to provide the engineer with a starting point.
We all know what a Wal-Mart or Home Depot looks like. When it comes to the area surrounding the building, this is where things must conform to the site purchased. The process being backwards, at least to us civil types. The footprint of the building is decided. How big a structure needs to be built to supply the anticipated traffic. From there, the civil engineer takes the job. There are fixed and fluid variables that enter into the site design equation, and many of them are decided by local codes.
- Drainage needs to be sorted first. Knowing how big the building is, how much of the project do we need to dedicate to drainage? Most local codes will let you work in cubic feet of storage, meaning deeper basins that take up less area. Some require a percentage of the site have allocated green space, which also can be retention basins.
- Number of parking spaces may be called out by ordinances. Handicap spaces are also controlled that way. This is the reason you may see a lot of accessible spaces in some parking areas and not many in others. Local codes adjust that. The only requirement is they do not allow less density than the ADA guidelines outline.
- Access is next. We would love every entry to have a nice long decel lane, wide access driveways, and plenty of entrances and exits. The site will restrict this as well as the roads they enter.
- The owner and engineers will need to look at the aesthetics and curb appeal of the project. Nobody wants to enter an unwelcoming site. Green space, trees, and easy access are all key.
- There is no such thing as a great parking lot design. All ideas have drawbacks. Make something easy to navigate and somebody will find a way to gum things up. Useable area has the biggest impact on one way or two-way lot lanes, nothing else.
With these less than desirable plans in your hands, you bid the job. The hard bid was won, and it is time to work. I have seen many a small to medium contractor do a bang-up job for an owner and get asked to do more on a negotiated basis. Keep that in mind when you look at this type of work.
My best advice is to submit your concerns right after being rewarded the job. You already have the list of concerns that was made during the takeoff, estimate, and bid where contradictions and confusing details were presented in the plans so this should be easy. Addressing concerns at the start will accomplish two goals, it shows that you’ve studied the plans well and gives you the chance to bail if you do not like their answers.
Change orders are normal. Do not hesitate to fully explain the problem and solution. Your submission will most likely pass through two or three people before it gets approved.
- Fully explain the issue requiring the change order.
- Propose a fix and the cost associated. Remember, never ask a question without providing your own best answer.
- Make your submittal stand on its own. In other words, if I read your change request and have any questions, you did not do your job well enough.
Site selection, design and construction are all handled by different people. Seldom does one person follow a project through. Make sure everything is written down. Communicate with all stakeholders so that there is no misunderstanding when things get handed off.
When you need assistance with 3D data for machine control and layout, surface and layout files, or point cloud processing give us a call. We can help!
There are many reasons you may be compelled to do a quick fix on a bad surface and get it in the field. We never use an engineer’s surface sent with CAD files because we are being paid to build it correctly. From a time-management standpoint, we can build a surface faster than the time it takes to break it apart, find the issues, and put it back together. When someone finally gets the files, and crews have been on the job for three weeks, a quick (and very dirty) surface turned around overnight is better than nothing.
Everybody talks about only sending out “perfect” models. My hat’s off to you, most jobs do not get enough warning to make them pretty from the start. I am speaking of the people in the office who get the call four minutes after the CAD files hit everyone’s inbox and the voice on the other end is wondering where the model is. That is the reality we deal with. Time to figure how to make this work.
The Surface Defined
When I talk about a surface, we are looking at the 3D elements that are elevated to make a model, contours, 3D lines, and points. Surfaces can be any combination of these three things. I will go through the advantages of each data type for a quick surface and what they do (alone or with other elements) to get you moving.
Contours are referred to as 2D lines because they are all the same elevation and still 3D because they are part of a surface. Contours undergo a lot of changes during data production but are often a quick way to get something out.
Initially, these contours do not look so bad. You need to trust me; they are a decent representation of the surface.
Creating a quick surface shows there are some problems. In this case we cannot send this out, work needs to be done. This may be considered work that takes too long for a quick start, but the spikes and bad information will cause more issues than they fix.
- Line density is all over the place. Some areas have long runs without anything and it is too dense in other spots. This is common for a civil 3D file.
- It needs break lines in order to make sense. A ditch is being blown over and slopes are not smooth and even.
Here is a screen shot of the improved density and some spike removal. At some point, you need to decide what a contour only surface should have. Here are some thoughts:
- This is for rough grading only. You will not get detail to get you within a half-foot, do not even try.
- Ditches and berms are easily flattened and there is no simple way to verify their existence. You need to go through the model and see if each high and low area look good.
- At the right is a mess that should be a nice retention area.
- Streets from contours are a hot mess. That may be fine for a new subdivision road but not for a rehab project or lane additions; those are usually close to grade to start, and you may make a bigger mess doing a quick surface. You need better.
- Linking contours with break lines can solve some issues. The problem is they take time to draw and auto functions can make the cleanup harder than just connecting the dots yourself.
In this image the problem is with roadway contours. The triangles will link in a bad way and make a surface that not only looks bad but does not perform. There are no real easy ways out of the issue except to connect them with 3D lines.
- Edge of pavement CAD lines can sometimes be elevated as they cross the contours. The straight segments created by the function can cause crossing line and new headaches.
- Any of the three lines that make up the road, (edge of pavement, face of curb, and top back of curb) can be drawn and then offset to get things closer. The problem can spike when elevations do not match between contours and 3D lines.
One can argue that it is just a starter file, and this is too much detail. However, this can become a problem when there is not much dirt to move, and crews need more exact information.
Contours have one elevation their entire length. 3D lines vary in elevation and are the best way to create curbs and other road and parking lot features. Files from engineers are usually not loaded with these lines. About the only time I see them are as break lines inserted to contain features that might have been blown over during design. Without any real 3D lines to look at or adjust, you will need to make your own.
Any time I go to the effort of drawing 3D lines, I make sure all the elements I am connecting are correct. Because of this, I only use them to tame bad spots in a quick model and not for beauty. More detail will be added later when we make the actual finished grade model.
After trying not to draw break lines, the field may finally compel you to clean things up a bit to get closer to the real finish.
- Building pads benefit from a few quick lines to make a fence around the single elevation.
- Retention areas can benefit from a little cleanup by installing lines on corners and bottoms. These are usually the first thing done on a site so that makes sense.
- Large sheet graded areas are the first to go as well. They are not too hard to do so you can get a jump on the field by making these look nice while drilling down when building the hard stuff.
- The retention to the right looks better with some break lines. It still needs some more and the northwest corner is in the dumpster.
When doing a quick repair on a bad model, points usually never enter the picture. There are times when they will come in handy though.
- When you have utilities to install, structure elevations shown as center.
- Any existing utilities and bends that can be noted by points can save a mess. 2D or, 3D is better if possible, will come in handy in the field.
- If you have control points, include them on the screen as well as a text file to upload to the data collector for an easier calibration.
Sometimes you won’t get the pieces used to make up a surface, you just get the triangles. With Civil 3D files opened in that program, the individual parts can be extracted as well as other important data to make the job of the model from the engineers clean up easier. When all you have to work with is a mass of triangles, you do not have a lot of options for greatness.
- Trimble Business Center will keep your changes to 3D faces when regenerating a surface.
- Carlson will make a new model; preservation of swapped faces is some work. It involves saving your changes and incorporating them into the new model.
- Do not be worried about removing faces and adding break lines to an engineer’s surface. Civil 3D has a lot of settings, usually CAD technicians get their finger stuck on the “make a lot of triangles” button.
- The surface on the right is not too bad. There are areas that could be improved. This may be okay to start the job but will have to be improved to make the grade.
- Here is that same general area as a finished grade model ready to work. Better triangle density and smoother contours on a gut check let us know this will do the job.
I performed a takeoff from an engineer’s surface that consisted of 3D faces against our model. Here are the results:
- This is a small area at about 3.5 acres and there is a lot happening in a little spot.
- There is a couple thousand-yard discrepancy in the dirt numbers.
- The max cut and fill is 4-feet each. Their model is considerably different than ours.
“We need it now” is all too common with the fast-track world we live in. I do not want to see people waiting to deploy technology on a job. I also do not want to see dirt moved twice because somebody got a model that had too many issues. A balance must be struck. It is our job to be sure that what goes out quickly is not going to cause extra work. Be careful and check what you are sending. It is better to have the field complain they have no model than a bunch of rework.
For this article I will go through the evolution of the way things were to the way they are today when working with local coordinates. The information is presented as fundamentals to your understanding of how GPS works on a jobsite. Not being familiar with these concepts could cause issues along the way as you work through working with local coordinates.
Talking Survey Coordinates
When performing a layout and surveying with a total station, level, chain, or a combination – equipment jobs are assigned coordinates based on a random northing and easting. For smaller sites, it seems the most common starting coordinate is 5000, 5000. The “bottom, left corner” would be the starting location of the assumed coordinate system so the numbers going to the north and east would get bigger. This also leaves room to the south and west of the start point to avoid negative coordinates – more on that later.
In the past, points that were to be laid out on a job were assigned coordinates that conformed to where the arbitrary coordinates started. This system works well, but we haven’t used it in years. Almost all our GPS based jobs are located on state plane coordinates. Later in this offering I will talk about the mechanics of a localization or calibration. It’s the same thing but different names depending on the brand of equipment you use.
In the early days of GPS survey, localization was carried out by aligning GPS coordinates with the local grid assigned usually during the initial survey. Things worked well and the equipment wasn’t confused by the numbers. Somewhere along the way, people realized that their data could be monetized if it could be placed on a map in the correct location. A surveyor working in a town or city could use the control from surrounding jobs to locate control for a new job. That information would be worth money to both the city and GIS providers to enhance location services.
The shift to map coordinates was not immediate. One of the reasons was surveyors did not want their control being used for a neighboring job that they were not being paid for. Many also stated the additional liability of somebody using their state plane points and then something going wrong. It may not be their problem but that would be decided by lawyers, so it was best avoided.
Localization and Calibration
Autonomous GPS receivers used in cars and hiking units are incredible. We can geolocate within a few feet. This was not always the case. Selective Availability (SA) was an intentional degradation of public GPS signals implemented for national security reasons. In May 2000, the U.S government discontinued its use of Selective Availability to make GPS more responsive to civil and commercial users worldwide. When SA was in use, the best autonomous units could achieve was 50–100-foot accuracies. Great for travel or hiking, not so much for precision. Without doing a deep dive, we employed a base station to triangulate signal locations to give us the accuracy we see today.
How do we get the GPS to see us in state plane coordinates? A localization is required to sync these two different measurements. What you are doing is telling the GPS, “When I am at this Latitude, Longitude, and GPS height, tell me I am at this northing, easting, and elevation.” There are a couple of things to note in this statement:
- Latitudes, longitudes, and height are what the GPS is reading all the time. Our screen shows the coordinates we want it to.
- The units all use meters in the background, converting them to decimal feet at the end for viewing on the screen.
- It is important to note that I used the word “height” when talking GPS and elevation with state plane coordinates. GPS uses a mathematical ellipse to establish the not regular sea level. Your elevation will always be different, sometimes over 100 feet. Be sure you are talking about the correct type of tall.
Above is a shot of a few lines of a site calibration. This is the Trimble data collector file, (.dc)
- The origin Lat/North are in the first two rows. This is how we tell the GPS to give us desired coordinates for latitudes and longitudes.
- The horizontal adjustment is needed to force the points to match up. The translation and rotation of the points will usually never be an issue to you. When things do not work right, then you get deep into this data.
- Scale factor is usually the only thing you need to pay attention to. If you have a 1 followed by four zeros or four 9’s following a decimal point, then you should be good.
- The GPS must be mathematic, just like the ellipsoid used for mean sea level, your job gets tilted on an inclined plane.
- The vertical adjustment shows the slopes to the north and east from an origin point in parts per million. In this case, the slope to the east is .000058%. This is the reason you need to have multiple calibrations on longer jobs. In one mile, the vertical on this job could be off by three tenths.
Here is a control file from a Topcon controller. I drilled down to control point 120 and was given the following information:
- The northing, easting and elevation of the point are either keyed in manually or imported from a text file.
- Upon occupying the point, the relationship is made.
- The residual values, (accuracy) are within my desired tolerance so I checked both the vertical and horizontal use in the control calculations for this point.
- Note the almost 70-foot difference in the local elevation and the GPS height.
Things do not always go according to plan. When something does not look right, it is probably wrong.
US and International feet
A source of confusion and sometimes great expense is that in the United States we use two different measurements for a foot. The difference is 2-feet per million feet. The problem is that state plane coordinated can be in the millions so a mix-up can put you several feet from your intended target. Six states use the international foot with the balance using U.S. feet.
States using International Measurement are:
- North Dakota
- South Carolina
To make things even more complicated, the U.S. foot will be deprecated as of January 1, 2023 making the international foot the standard. There is a lot of commotion going on around the proposed change and as far as I can tell, it will happen. The biggest argument is that a majority of people and real estate use the U.S. foot, why not change to that? Time will tell, stay tuned.
A fun website
To help you navigate to any place on earth (and your big jobsite) easily, I have been using an app called What 3 Words, https://what3words.com. The world is now broken down into 3-meter squares. This has really helped emergency services as well as friends who can never quite get directions right. We use it in the field to tell people where we are meeting. It sure beats “go to the job trailer and keep driving.”
Over the past 15 years we have had to sort through different types of 3D data for a job. In this presentation, I will go over some issues we find when dealing specifically with roadway rehabilitation. Issues can come up with any type of construction data. However, roadways are more critical. Maintaining traffic and smoothness requires higher precision than a new parking lot for a retail site.
The major interstates are in place, but in different areas they are getting widened and constantly repaired. Larger arterial roads get re-routed and widened as population changes occur. These and other factors mean that you need to be efficient at this type of work as there is a lot of it out there.
In our hypothetical case, we are milling, filling, and widening a road section. Here are the various types of data you could receive for existing conditions on a job.
GPS Rover Shots
The project has been worked on for a while and uses GPS to get things organized. The job is calibrated, and work has been done in the field. Some topo shots have been taken of the edge of pavement, so the new lanes have a vertical to attach to. The main issue here is the quality of the data.
- How good is the calibration?
- Did the operator check the known control?
- How long was the point occupied? In heavy highway work, long observations are dangerous.
- Was the pole plumb?
- How recently was the rover pole calibrated?
There is no way to know this information by reviewing the metadata of a shot or a session. No assumptions can be made regarding quality. Hopefully, there is an opportunity to make a surface of the data to check accuracy. Yes, there can be issues with the GPS performing the check. Where precision is an issue, I do not fully support GPS only information.
GPS information gets used a lot, and we work with it all the time. No need to stop trusting GPS rover data. You do need to be aware of the problems you may encounter when counting on it for high precision. Fully trusting GPS data for a highway reconstruction will most likely not get a rideability bonus.
I have been doing topo work with drones since the infancy of using drones to collect data. Drones are a great tool but must be used properly. In addition, expectations must be managed to provide accurate information within tolerance.
The best accuracy one should expect from a drone topo is a half a tenth (.05’). That is great news for doing interim dirt topo(s) and pre-job topo(s) on mostly cleared land. Photogrammetry software does not perform well with a single-color surface. We often have problems with sand and dirt work that is smooth and monotone. White and asphalt paving both have the same issue. It is difficult to get any consistency with photogrammetry on these surfaces. Things get worse when you try and extract curb data, elevations jump. Photogrammetry cannot be used to obtain paving elevations for rehabilitation and widening.
Over the years we have provided drone topo(s) on heavy highway projects. That information is used as part of a hybrid surface. The drone is great for slopes and median while a higher accuracy method is employed for paving areas.
I am excited about the use of drones for LIDAR (Light Detection and Ranging, 3D Laser Scanning). We have relied on full scale aircraft mounted LIDAR for years and are happy with the results. The problem with drones and LIDAR is vibration. The measurement is taken and by the time the unit gets a return, the vibration has put the unit in a different location. Yes, the change is small but enough to cause inaccuracies that are being improved upon every day. I have visited some white papers on new offerings and am confident the issue will be sorted soon. I feel within a year, the process will be accurate and repeatable. Within five years of that prices will drop so this will become the norm.
Many of us have seen mobile LIDAR units at tradeshows. The late model SUV with a GPS antenna and a big scanner mounted on top is impressive and expensive. I get a lot of calls and emails on this topic, and the manufacturers know this needs to be solved.
Mobile LIDAR is a great solution, but the issue is in the cost. Figures can vary based on options and software but $750,000.00 is a good estimate all in. That’s too much money for a contractor, so leaving it to survey and mapping companies is a better option. I have consulted with numerous firms considering the purchase of this equipment. Here are the facts.
- It is expensive.
- There is a steep learning curve.
- People will need to be hired to operate and maintain the gear and vehicle. Usually two expensive office/field operators.
- To gather and process the huge amounts of data collected, you will need:
- Expensive field laptops
- A lot of solid state portable hard drives. This data is easier to express mail than upload in many cases.
- An office computer(s) to do initial processing and reduction to specified densities. This takes a long time and can use up to three powerful desktop units to keep various jobs on schedule.
- Finding people to buy data. This unit needs to be moving all the time. The crew will be on the road a lot. Depending on population density, a 500-mile radius may be needed to make this profitable.
As you can see, quality data comes at a cost. When hiring the service to be done for you, expect from 4-10 thousand dollars a mile depending on the job and distance from the unit’s home base as well as the project size.
Ground Based LIDAR
Currently, the use of this technology is striking the balance between accuracy and cost, and it comes with a price. Ground based LIDAR can be as accurate as necessary for paving work. As with any technology, especially one we are expecting high precision, conditions need to be met.
- The job needs to be localized and the quality of that must be verified. LIDAR needs to be setup over known points. This is not necessary for all scans, as they will register to each other. On any linear project like a roadway, there needs to be a high number of good control points for verification and setup.
- Operator error is the main issue we see for poor data. It is easy to use but setup and operation are procedural. If one step is missed in the process the data quality will suffer.
- These instruments are sensitive and need to be handled carefully. If a drop or a fall happens and the unit is not checked, data quality can suffer.
- Training is not too difficult. One person can do the work and needs to be responsible for the unit.
- Prices are dropping to the point where a contractor can pay for a unit with the profit from a job or two. Rental is an option as the dealers know you need it and may not want to purchase.
- File sizes are going to be big, so be prepared for a lot of storage space.
- Scan only what is needed. We see a lot of scans including trees. We are doing roads, not lumber.