Several years ago, I figured out a way of doing a site calibration without ever actually doing a site calibration. If I have a job in Access or Survey Controller that is State Plane, State Plane with a surface adjustment factor (TxDot), or anything with a Geoid, I can do a calibration in the data collector creating a simple projection without a geoid that will work in SCS900 or for Machine Control.
The problem was actually explaining to someone how to do it. Several months ago someone was asking how to do it. I thought I had written a step by step explanation but no. What I finally found was before I started doing it this way.
Last week I had to do one for a job that was about 300 miles away and never left my office. I downloaded the plans, setup the job in Survey Controller, and then did the calibration. As I did this, I took pictures and wrote notes.
Here is the write-up if anyone is interested.
James
I'm going to get some popcorn....
I've been told by machine control people that they can't use a state plane projection or any other projection to do their work, they can only use a calibration. Essentially, you are calibrating a state plane projection so it's a state plane projection, seems?ÿa lot easier to just use the projection and save all the steps, but I guess if the machine guys can't use that.........
Back in the old days of GPS we would do the same thing, but we weren't doing it for the XY, we would calibrate, but we were doing it for the Z cause Geoid models were bad and calibration was a way to kinda get GPS elevations "close".
We had one job near Wichita Falls in 2013 that was a calibration but still had the geoid/inclined plane in it. The guy doing the model had to break it up in about 1.5 mile segments or it would fail to load in the dozer. The next job on that same highway, I had figured out most of what I posted and we were able to load the whole 12 miles in one file. I never tried just State Plane. Almost all of TxDot is State Plane with a surface adjustment factor and the dozer doesn't like that either.
James
We had a mine site that calibrated to the old control, it was messy, the control was from the 70's and who knows how much blasting and high walls had shifted everything. The machine guys would only calibrate, they said how good that works on their other mines, which are all in flat "quiet" Geoid model areas. This one is in hilly almost mountains land. Well they had a road to build as another access that went almost 12 miles south outside the calibration. They were 6' off by the time they got to the end, which of course is a lot of dirt.
Sounds like a paper calibration to me.?ÿ Are you the one responsible for the results? What happens if the paper calibration is wrong...do you just double check the coords and call it a day??ÿ Seems like a recipe for disaster to me.?ÿ
The calibration is just as good or just as bad as the projection you started out with. If you have junk to start with, you will have junk to finish with.
If you have junk in your data collector and set 4 control points, then the construction company calibrates to those 4 points, they will still have junk. This is no different except you are removing the small error that is potentially in every GPS shot.
I am a salaried employee of the construction company. My liability is their liability. I don't worry about liability, I just worry about getting the job done the best way possible. They do have the right to fire me anytime they want.
James
It makes sense to me that a calibration is only an attempt to develop a projection.?ÿ Why not take it straight from the horse's mouth? Pairing up lat-longs with their grid positions doesn't seem like a recipe for disaster to me.?ÿ?ÿ
My understanding of the need for calibrations for machine control has to do with establishing a plane for the vertical.?ÿ I've been told by smarter fellers that the math to do geoid calculations is too complex to be done in real time by by the big machines.?ÿ
Several years ago, I figured out a way of doing a site calibration without ever actually doing a site calibration. If I have a job in Access or Survey Controller that is State Plane, State Plane with a surface adjustment factor (TxDot), or anything with a Geoid, I can do a calibration in the data collector creating a simple projection without a geoid that will work in SCS900 or for Machine Control.
The problem was actually explaining to someone how to do it. Several months ago someone was asking how to do it. I thought I had written a step by step explanation but no. What I finally found was before I started doing it this way.
Last week I had to do one for a job that was about 300 miles away and never left my office. I downloaded the plans, setup the job in Survey Controller, and then did the calibration. As I did this, I took pictures and wrote notes.
Here is the write-up if anyone is interested.
James
Seems to me all you are doing is taking the word of whoever set the control for the project that it's right.?ÿ Maybe it's different where you are, but about 75% of the jobs I've worked on in the past few years, jobs supposed to be on SPC, are not where they say they are, from 1.5' to 100' H, from 0-6' V, just on recent projects, but they were used for the design and have to be used for construction.?ÿ How do you check for that with your method?
Everyone has good points, I've heard for a while that the Machines can't use a Geoid, which doesn't make much sense because a Geoid model is a file that adds, or subtracts to an ellipsoid height at a?ÿparticular lat, long to calculate the ortho height. It doesn't seem to take much room or use much computing power with modern day GPS gear.
Using an office calibration with no connection to coordinates on the ground?ÿwill work fine if the control on site matches. If not then the calibration will be of no use. If you calibrate to state plane numbers and use a Geoid file to match elevations with heights, and the control was done with GPS using these same parameters then good. It doesn't matter if the on-site control is "off" as long as it was laid out with spc parameters. The thing to understand is that designed buildings, curb and gutter, sewer, all that stuff was designed off something, it's always best to get on that something, regardless of what the black box says. A finished floor that is staked off even a little can cascade through the entire job. I've seen lots of concrete ripped out for a couple of hundredths of a foot.
That has already been done by me or someone in charge. If the file in Access or Survey Controller is junk then the box calibration coming out of it will be junk. You cannot calibrate the junk out of a file, not today, not ever. You have to have a file that you trust to start with.
In my example, there are only two control points and a road profile. The Project Manager has already had his surveyor check everything but when the surveyor tried to calibrate it to have something to export, things got funky. He didn't elaborate but calibrating to two points did not work. They found this by checking. He will take my calibration, set up on one control point and shoot the other. Shoot the start and end of the profile. And then start building a road. I have worked with him for 15 years. I trust his work and he trust mine.
I can show you plans on a job that the GPS points, the main control points, are in State Plane. The control points in between, traverse points, are modified State Plane. The alignment is modified State Plane. The GPS points disagree with everything else by about 700 feet. Have you ever tried explaining scale factors to an Engineer? The GPS points were stamped by a Surveyor that is currently on our State Board. His work was done right but was misused in building the plans.?ÿ I told the TxDot engineer I would fix it and I fixed it. All this should be done before you do a box calibration.
The job I mentioned near Wichita Falls had a very good survey company from Wichita Falls set the control several years prior. A different company (state wide) was doing the design survey and convinced TxDot the control was bad and set new control. It had a rotation of about 3 feet in 7 miles. That is why I calibrated to their control but kept the Geoid. It worked fine in my data collector but was not worth a crap for machine control. That is what eventually led me to developing this method. I kept two job files on this road, one for anything that had to fit the road and one for anything that had to fit the right-of-way. If you come across a job near Wichita Falls that the R/W does not fit the new road built, that is why. It is about 3 foot off on the NE end. I explained it to the Engineer and he said do it. All this needs to be resolved before you do a box calibration.
Geoid grids are 1 minute of latitude and longitude or roughly a 1 mile square (very roughly). The only way this method does not give you what you started with within a very slight tolerance would be a having an area covering several 1 minute geoid blocks that have a significant grade change. That is the reason for the extra points. I have yet to see an unacceptable difference in the geoid in the flat land where I work.
We are currently working on a job that the GPS control is so old, there is no geoid. After I tied all the control using a geoid, I could see a slope of about 0.7 feet. I took out the geoid and adjusted my network vertically until it fit. THEN I build a box calibration.
You said "Seems to me all you are doing is taking the word of whoever set the control for the project that it's right." That needs to be done before even thinking about building a box.?ÿ That is not what this article is about.
James
I was basing my comments on this statement you made.
"Last week I had to do one for a job that was about 300 miles away and never left my office. I downloaded the plans, setup the job in Survey Controller, and then did the calibration."
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I was basing my comments on this statement you made.
"Last week I had to do one for a job that was about 300 miles away and never left my office. I downloaded the plans, setup the job in Survey Controller, and then did the calibration."
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Point well taken.
I didn't want to cloud the instructions with details about one particular job that nobody here will ever see.
James