The GPS grades we've been getting from the engineer are'nt very accurate, + or - 0.20 the locations like building corners etc are on just the elevations are inconsistant. Are you guys finding the same thing? DP
The Trimble 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. If his model is not accurate, then the job will be off. In other words, the finished product is only as good as the data installed into your machine from the model. :cool2
The engineers grades are off at the stakes. We have a series of 10 pads 50'X 480' all supposed to be the same finish elev. Hub elevations don't match , what a pain in the ***. Need a new engineer I think. DP
gps grades I have ran into that problem many times before, and until an incident last year, my boss thought that I was just being difficult and having trouble with grades. They (survayors) came back to replace some grades around a building for a side walk. 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 three on the last ones. the survayors said that they did not do the first layout and that that guy make the mistake. HIS gps was right. . After arguing with them, the building Super set up his old fashond 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. Im not sure what happened in the end, but I do know of a building that was done, and the floor grades that were used were actually the parking lot grades. the building is now too low. The problem is that some of these guys put too much trust in there machines, and even when you can see the error, cant or dont know how to correct it. the machines are only as good as the person that entered the information. so if there is an mistake it carries on through the job There has been many times I wished for a survayor/enginer that knew what a level and chain were as well as how to use them. sometimes even a simple string line would be better.
Do you have your base set on a tripod or a permanent post? I know, it shouldn't make a difference. But on the smaller jobs when I'm setup 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. I'm only saying maybe .05' to.10' at the most. Also, how many points did you calibrate the site with? More does seem to be better and especially if there is a good difference in elevations. I assume you do a have a GPS equipped machine to grade the pads, right? If you are just grading level pads all you have to do is program and enter a new level into the machine once you find the proper elevation. I program in my own levels and slopes probably 90% of the time in the field, but it's nice to have a design when there are a lot of cross slopes. But even with a design there are usually mistakes. I'll just shut the automatics off and do it manually using the elevation readout on the screen like a grade-eye. Sometimes you have to do a little math but I keep pad and pencil and calculator in the cat with me, it's faster and easier than arguing with an engineer.
We are using a grader with Topcon and a laser to finish, the problem is what stake to match because the grades which are supposed to be all the same finish elev. don't match. In checking from stake to stake with a grade rod the elev. are + or- 0.20 so which stake to tie into? DP
DPete, If grades are involved I insist on a solid bench mark with an elevation that they will sign off on being 100% right. Off of that I might set more bench marks where I need them. If I have one bench mark I know is right I can check there elevations before if it seems off, during if it starts looking wrong, before I do the work to finish it, and afterwards if it doesn't match there stakes and I want to show someone. Kent
I have a real battle when building walls to get my guys to go back to solid datum every day and setup properly. You will notice those variances each day because of the accuracy of the laser....predominatly affected by the distance you are working at. It helps to fully understand the capability of your laser. When working at distance lower the staff from high until you hit level and make a mark. Then raise the the receiver from low until you hit level and again make a mark. You may find a 1/4" difference at the longer ranges of your laser. In between the two marks is your level. This is because the laser beam becomes broader the farther it travels....a bit like a spotlight or headlamp. That said the main thing is to have that solid datum so that you don't have compound errors.
Yes, like a BM in the road centerline. That's the reason for this thread, It takes alot of time to check the engineers work, if they can't do any better with GPS maybe the finish grades should be shot with a transit. The locations are dead on just the elev's are off. DP
Sounds like incompetent surveyors to me. I agree, find out what their benchmark is and you check their grades. Easy enough to do with a laser. I don't trust any grade stakes until I verify a few are correct myself. If the pads are all the same elevation, don't all you need is one hub?
Right, one hub will do it on this job however some of the sub-division jobs might have 150 pads all different finish elevs. It would be alot easier and faster to be able to trust the engineers grades, nuff said. DP
GPS Errors Hi, Folks. This GPS stuff is all new to an old dinosaur fossil me. I got to use GPS for the first time last Thursday. It was in a D8N dozer and it went down at around 8.30 am and was not repaired until 4.30 pm, a bare 1/2 hour before knock-off. In between, I was running as I have for over 40 years, working with a few good, old-fashioned grade stakes. I got to use the GPS for maybe 2 hours out of a 9 1/2 hour day. Great initiation, I thought. However, I do have to agree with the posters who reckoned that having a good solid datum to work from is a BIG help. Having a good laser unit or a plain old dumpy level and staff on site to check levels from that datum is just about a must. Having some-one who can - and will - use the old-fashioned gear is a huge bonus. I spent 7 years working for a contractor who did almost nothing but level house pads ready for building. We did up to 600 pads a year for just one builder, never mind all the others we worked for. We did all this work with laser levels, an emitter set up on a tripod and a sensor set up on the side of the cab/canopy - NOT on the bucket or blade. Very rarely did we ever get a job where there was not a pre-set datum. Sometimes though, it was so far above or below the level of the job that we had to transfer it to some place a little more user-friendly. In these cases, I always took the time to set up a good solid datum to work from. Even on the jobs where there was a user-friendly datum, if I couldn't finish the job that day, I would set up a second datum to check back from when I set up again the next morning. Since I always kept my calcs from day to day, double checking these 2 datums always showed up any errors that I may have made. The first law of computing has ALWAYS been GIGO - garbage in - garbage out. If the surveyors, engineers, draughtsmen and/or programmers make a stuff-up, the poor bloke on the bottom of the food chain out on the job has NO chance of getting it right. Just my 0.02.
I am new to the forum but I surveyed for 12 years before getting into the construction business, GPS will get you alot closer horizontally than vertically anyday, everything depends on your satelites and how good the control you initially came off of to set up your base station. You should always check your elevations every day especially if you have machine control. I always set at least two BM's in concrete to be able to go to with your stake out rover or the blade on your machine to check you are the same. The bottom line is GPS just isn't as accurate as a good old fashioned level
Welcome to HEF tripod58! :drinkup
Vertical levels with GPS. Hi, Tripod58. Welcome to this little corner of the world. As mentioned above, I got to use GPS for the first time 2 weeks ago - and only got to use it for about 2 hours out of the day due to it malfunctioning. Since I posted the message above, I have spent a week on that machine and now know a LITTLE more about. As you mentioned, vertical position is not nearly as accurate as horizontal position. I would not want to be trying to do final trim with the unit that I was using. Put the blade on the ground and just sit there and the level would constantly change by up to about 200 millimetres - 8 inches - in extreme cases and even when it was having a good moment the level would vary rapidly through a range of about 70 - 90 millimetres 3 - 3 1/2 inches. The boss is happy if that dozer can cut to +/- 40millimetres - almost 1 3/4 inches each way.
If you purchase a GPS unit and expect dead-nuts on accuracy you will be sorely disappointed. It works great if you have a big cut or fill, so you don't have to work around grade stakes or survey it multiple times. That's why we chose to mount it in a dozer and not a blade. Just hog the dirt in with the dozer, get it within a tenth or so, and then strike it off with the blade. You have to blade it anyway to get rid of the cleat marks, and it leaves a better, smoother surface to compact. I always have a hub that I set my blade on to confirm my elevations are correct a couple times a day. I know that seems like a pain, but even so the GPS is still many, many times faster and more efficient than having to check grade manually or even a laser equipped machine. It just takes time to learn and get used to the little quirks with it.
Faster Than Laser??????????????????????? Hi, Pushcat. From my strictly limited experience of GPS, I would agree with everything you said, except that GPS is faster than laser. Even then, I would agree if you have multiple levels to cut or fill, or more particularly if you were cutting new landscape levels as in sub-division work. However, I'd back the laser in every time with a competent, laser-experienced operator on a single-level pad, even if it has crossfall. (Just use a dual-grade laser.) The reason the laser is faster is that it doesn't have the built-in error or variation that GPS has. The lasers we were using had +/- 1/2 an inch tolerance on the 'on-grade' indicator and it wasn't hard to keep the 'on-grade' light flashing all the way across a pad, even with the dozer. I spent 7 years using laser with both 4-in-1 equipped track loaders - 941's. 943's and 953's - and with a wide gauge Cat D5B dozer with a normal bull blade. I was cutting house pads to under +/- 3/4 of an inch - 20mm. I had been with that contractor for about 2 years when he equipped all the track loaders with spreader bars, just like the ones that skid-steers use but on steroids. That made it a LOT easier for them but the dozer didn't have a 4-in-1 bucket to pick a spreader bar up and hold it, so I had to continue doing it with just 'ye olde cutting edge' - I ran the D5B for over 2 of those 7 years. Even on split-level pads, the laser is still quick and far more accurate than GPS for grade. Sometimes, you only need to move the sensor up or down the mounting. Other times, you will have to reset the laser emitter to a different grade but that is not hard if you have a good benchmark to refer back to. A GPS/laser technician told me last week that there is a new system coming on the market now that uses GPS for horizontal location and multiple laser beams for vertical monitoring. The GPS picks the appropriate laser beam for the current location of the blade. Now that will be interesting.
When we first got setup with GPS we mounted it on our D6M because that was our newest dozer we had on hand, that was 4 or 5 years ago. The salesman at the time said Trimble/Cat was developing a GPS/laser system but it wasn't quite ready to market yet. He said in a year or so it would be ready and we could add it on to our system for around $10,000. Well, 2 years later it was ready, but for $25,000 and not the simple plug in add-on, but a major rewire. We didn't do it because we plan on updating with a new and bigger dozer with GPS. The D6M is a small dozer, but it sure works nice for fine grading, but than again with a bigger dozer you get more production. I went to GPS training 2 years ago and they had a D6N with a GPS/laser setup, so that has been on the market here for a while. I haven't seen any out on the jobs, but around here even GPS equipped machines are rare. Probably 98% of the work I do is with a laser. I always keep a laser and a grade eye with me. Most of the dozers have grade eye rods mounted on them, and I have a rod that I keep in my truck to mount on the loader arm of whatever 953 or 963 I have on the job. Today I was building a street in a new subdivision and used what I think is the most reliable way to check grade, a four foot level and a ruler. All I had was a 953.
Laser Rods Hi, Pushcat. The employer mentioned in my post above did not have the laser receiver mounted on the bucket or blade. He had vertical rails made of 2" square RHS bolted to both doors of all the cabs. These rails had survey staff scales stuck to the outer face of them at the appropriate heights and the magnetic-base sensor was stuck on the rear face of the rail, using the staff scale to set the working height. His reasons for doing it this way: * It did away with any errors due to cutting edge wear. * It did away with any errors due to not getting the bucket back to the same angle every time. * It allowed for different operator styles - some operators using 4-in-1 buckets trim with the bucket fully open and laid down flat with both bucket floor and dozer cutting edge on the ground. Others, like me, only open the bucket marginally and trim with it in a vertical position, rather like a dozer with a skid plate. This method alows you to clam up any scraped material if needed. I'll only lay the bucket forward if the material is too hard to cut with the bucket upright. The laser sensor would probably have worked better fitted to the blade of the dozer as there was a problem with accuracy if the ground was hard enough to cause the dozer to walk on the grouser tips instead of sinking fully in to the pad level. I used to overcome this by ripping any hard areas before trimming. I never told the boss about that trick. He figured he knew more than I did about running dozers so let him figure it out for himself. Well, I guess he figured he knew more than me. He often told me how to do a job. Funny thing was his ways never, ever, worked as well as mine, at least for me, and I reckoned that, since it was my ample butt in the seat, what counted was that it worked for me.
I see this thread made Construction Equipment magizine. June issue page 24 He!! we're big time. DP
The OTHER GPS Thread Hi, DPete. Did anybody tell'em about the OTHER GPS thread on here? From what I can gather, there seems to be a BIG difference in grade accuracy potential between GPS-controlled systems and GPS-monitored systems and I suppose that is only to be expected. However, it would certainly help if the grade display was a little less wildly erratic than the one on the GPS-monitor system that I used - like 4 and 5 changes per second with the changes ranging anywhere up to around 8" to 9" but more commonly 2" to 3" up or down. And that was while the dozer was standing still with the blade on the ground. There's nothing quite like a bit of a challenge though!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!