GPS Grades a Little Disappointing

Staff | September 28, 2010

User #1: GPS grades we've been getting from the engineer aren't very accurate, + or - 0.20. Building corners, etc., are on, just the elevations are inconsistent.

User #2: GPS I have used has been right on. The most important factor is the model that you run off of. The guy on the computer end can make you or break you. In other words, the finished product is only as good as the data installed into your machine from the model.

User #1: The engineer's grades are off at the stakes. We have a series of 10 pads, 50Œ ~ 480Œ, all supposed to be the same finish elevation. Hub elevations don't match.

User #3: I have run into that problem many times, and until an incident last year, my boss thought that I was just being difficult and having trouble with grades.

The surveyors came back to replace some grades around a building for a sidewalk. Only a few stakes were missing so you could compare it to the rest and see things were off. The new grades were out by 8 inches on one stake and then went down to 3 on the last ones. The surveyors said they didn't do the first layout and that that guy made the mistake. The building Super set up his old-fashioned level, and after going off the floor grades told them THEY were wrong and that they had better learn to use the tools they had.

The problem is some of these guys put too much trust in their machines, and even when you can see the error, can't or don't know how to correct it. The machines are only as good as the person that entered the information.

User #4: On the smaller jobs when I'm on the tripod I can tell a slight difference on my final grade from day to day; on the big jobs with a permanent base set it doesn't seem to be as bad. Also, how many points did you calibrate the site with? More [calibration points on site] seems to be better, especially if there is a difference in elevations.

User #5: If grades are involved, I insist on a solid benchmark with an elevation they will sign off on being 100-percent right. If I have one benchmark I know is right, I can check elevations before if it seems off, during if it starts looking wrong, before I do the work to finish it, and afterward if it does not match their stakes.

User #7: The first law of computing has always been GIGO: garbage in, garbage out. If surveyors, engineers, and/or programmers make stuff up, the poor bloke on the bottom of the food chain out on the job has no chance of getting it right. is a user forum where professionals in the heavy-equipment industry can exchange ideas and post questions/comments. Users include owner/operators, operators, company owners, technicians, and others. Posts have been edited for clarity/content.