I would like for some of you GPS Gurus to explain to me how to do property line surveys in the woods under canopy with any degree of acceptable accuracy.
1. Is it accurate?
2. Are the readings repeatable?
3. How consistent is the data?
4. Can you replace property corners with any accuracy?
5. Is this a magic box?
These are just a few of my concerns.
We have a Trimble Pro XR that is over 10 years old that we use for wetland surveys and I have seen it get to within 1.0 feet with shots and they are repeatable if you use the correct methodology (shoot the same control points before each session and post process the readings just to name a couple). Of course that is only for horizontal accuracy. The elevations are too rough for topographic mapping.
Oh, Merry Christmas to all of you guys and a prosperous New Year!!
> I would like for some of you GPS Gurus to explain to me how to do property line surveys in the woods under canopy with any degree of acceptable accuracy...
IMO, you can't do that. Forget it.
With that said, my local Leica salesman was in the office here last week claiming that our GS15's will be able to do some topo under canopy after the next firmware update but I'll believe that when I see it.
don't believe the salesmen and don't believe your GPS.
i have found that under canopy (leaves on the trees and mixed hardwoods and pines) you can not get reliable and repeatable solutions. you may get a fixed solution, but if you traverse conventionally to that spot you will see the errors (2-20').
if you are in the hardwoods in winter and all of the leaves are gone and you have minimal pines around you can get good data (with GPS and GLONASS). but like anything if i am going to trust it for boundary work i like redundancy. shot it at 10am and then come back and shot it at 2pm - for example.
all of that is out the window with the proXR. that is a submeter unit. it should never be used for anything related to a boundary other than recon. if you are going to try GPS in the woods you better wait till winter and you better have a centimeter unit.
If you mean a real canopy, like those on car lots, or Christmas tree lots, you can survey under them with no loss of accuracy. But you probably mean tree canopy, and that varies. Pine trees are the worst offender, but all trees with wet leaves are a problem. If the trees aren't that tall and you have a slide pole, you might be able to get high enough for a clear signal. Generally, though, even trying to get shots around trees is very frustrating as you gain and lose lock and stand there waiting and waiting. I wouldn't do it. If you try it you will swear never again.
We do some boundary work under canopy. We have newer equipment, and only do it with an RTK fix. Whenever questionable, we shoot a pin in the woods, lose "fix", and then stake it out again. If the results compare favorably, I will accept the measurement.
That being said, whenever there is thick canopy, we suck it up, and use the robot.
> I would like for some of you GPS Gurus to explain to me how to do property line surveys in the woods under canopy with any degree of acceptable accuracy.
>
> 1. Is it accurate?
> 2. Are the readings repeatable?
> 3. How consistent is the data?
> 4. Can you replace property corners with any accuracy?
> 5. Is this a magic box?
>
> These are just a few of my concerns.
>
>
> We have a Trimble Pro XR that is over 10 years old that we use for wetland surveys and I have seen it get to within 1.0 feet with shots and they are repeatable if you use the correct methodology (shoot the same control points before each session and post process the readings just to name a couple). Of course that is only for horizontal accuracy. The elevations are too rough for topographic mapping.
>
> Oh, Merry Christmas to all of you guys and a prosperous New Year!!
1. Define your accuracy requirements. For me, no.
2. Define your limits of repeatable. For boundary surveying with monuments, no.
3. Define your limits of consistency. For me, yes, it's consistently off what I would expect.
4. No.
5. Yes, there is magic, and just like magic, there is slight of hand and if you don't know how the trick is supposed to work, it will look really cool, but in fact, you'll fall through the trap door.
I love my Pro XRS for mapping rivers, creeks and other natural boundary lines. They are meander lines only, so it's not really an issue. For the reference corners that touch them, they either get the R8 treatment or total station.
You CANNOT use GPS under the canopy reliably, period. You may get some good shots. Right now, with the leaves off, it's MUCH more forgiving, but still will be wrong in some cases.
You have to define what your tolerances are. If you're happy with a foot to three foot of slop in each shot with boundary markers, hammer down cause you found your brand. If you actually want to have good survey measurements, put a pair out in the open and traverse in and then close out to another pair and adjust.
IF L5 comes online and L2C gets more birds up there, then MAYBE you will be able to work in the canopy. Glonass helps, but only where NavStar was sufficient. Should you try this, you better check, and recheck.
I have, in the past, when I needed ONE corner a mile away that was in the woods, would go and set the rover on it and let it log data for an hour or so, and then trim cycle slips and process that data. I've had the opportunity to then traverse by it later and actually tie it in, and it was 0.12' out. Close enough for me at the time, but on the edges of acceptability.
It's not the answer for everything.
We use conventional equipment (TS) in woodlands that have a distinct "floor" and "canopy" with trees above 25'. You can stand there all day long. You might even get a fixed solution. I was using the Topcons yesterday and got a "fixed" position, but the PDOP was over 11. Not a real reliable "fix". It's better done another way.
Brushy areas are the pain with the brush tops only 15 or so feet high. We keep the antenna high and clear a good radius out with the machetes. It helps to crank the elevation mask up, too.
You just have to take each site on its own merits...and check, check, check, and then recheck.
I'll pretty much echo what everyone else says. You can't get good shots in the trees. I've tried it, and if the tree cover isn't to thick and its winter, it will work. I've also tried it in heavy tree cover, and gotten fixed shots that were junk. You can just tell if it doesn't get fixed in a short period of time, the results are in doubt.
I don't use GPS under tree canopy.
I may use it for wetland flag locations, but even then its iffy.
gotten fixed shots that were junk
Ditto! I found way back in the 90's (the second day I had the GPS) that GPS "fixed" doesn't always mean "fixed". You need to check and recheck everything; particularly boundary corners. I have tried working under canopy and it's very iffy-sometimes it works, most of the time it doesn't. My advice is figure out just how much tree cover you can get under and have locations repeat-I think you will be disappointed. And yes, I have newer equipment now that will access all the satellites and it’s better, but I still can’t go under deep cover.
That's a common belief among non-Surveyors ...
Every once in a while some academic that has zero to do with surveying, mapping, or geodesy submits a paper to an esteemed peer-reviewed journal claiming that their new GPS gizmo WORKS UNDER TREES!
Each and every one of them is rejected for publication. I've done several rejections myself as a Reviewer for more than one professional journal.
The most common of these are Foresters with single-frequency receivers.
That's a common belief among non-Surveyors ...
Cliff,
What about tunnels and underground coal mines? According to some of the salesmen, these things work everywhere.
Dale Yawn
Savannah, Ga.
That's a common belief among non-Surveyors ...
I appreciate all the feedback from you guys. I had already made my decision about GPS in tree canopy. I just wanted to make sure that my thinking was correct. We have some local surveyors doing Boundary Surveys with their GPS and drawing plats and getting them recorded. I guess they think they know more that the Gurus do. Thanks again and I hope you all have a Merry Christmas.:-)
I do a lot of work in the heavy pine forests in my area. I use GPS in openings to reduce the amount of field traversing needed. However the openings need to be pretty good size. Even though a log landing or small meadow may seem large enough, I've found that GPS results are often not that tight due to multipath off of the tall timber that surrounds the site.
That's a common belief among non-Surveyors ...
When I saw this message this morning I thought to myself .... something is up. Terry knows GPS will not give reliable and repeatable answers under canopy.
The sad part of the story is the folks who really need to see this thread never will. They aren't paying attention like you and I do.
Happy Holidays.
Larry P
That's a common belief among non-Surveyors ...
Nebraska doesn't have the number of trees that most states have, but I did overhear a land surveyor once say that that they could get the RTK shot if they fixed out in the open and then quickly ran into the trees to get the shot before it unfixed. To each his own...
I wrote my MS thesis on gps under canopy. Give me your email and I'll send you a copy if you're interested.
It's entirely possible in a science-project sort of way. It really comes down to whether it is repeatable or not.
I was able to get repeatable survey grade results under heavy doug fir canopy. It was not efficient, and was not something you would stake from without a lot of post-processing and checking.
I wrote it up on the old Board so it might still be there, but here's what it took:
mission planning: perhaps most important. set your planner for a 25 degree horizon, then pick the times where the GDOP is less than 4. This gets you a roughly 45 minute window twice a day. Plan your day around it and be there set up and logging beforehand. You can always use TEQC or something to trim the data.
recording rate: 1s at bases and rovers. Also very important.
multiple bases: I used the WSRN bases and sometimes a Virtual Rinex with a very short baseline. (Sometimes this was the only thing that was both fixed and repeatable).
multiple occupations: otherwise how do you know it's repeatable?
I used a dual frequency receiver, will try the same method again with an L1 soon.
So yes, it will work, under limited circumstances and with many caveats. I did it on a few points that I couldn't brush or chainsaw and that I could not have efficiently traversed to.
The Boss scrutinized it thoroughly, and accepted it because it was repeatable AND it matched some modern traverse data by a tenth. Not saying you SHOULD do this, only that you can, and YMMV.
Dude! send me one too!
AS said above... we use a combination of GPS to set control pairs and a ground traverse between them them if we are staking a line, boundary, etc under canopy but we do recover existing corner monumentation under canopy using Static GPS techniques...and the thicker it is the longer the observation time...and you can be pretty confident after processing as you have control over your precision during the process. Multiple vectors from additional receivers is a plus.
These new BLK III satellites that start going up in 2014 will have a stronger signal and are predicted to penetrate dense foilage and they are talking a usable constellation of 70, 80, or 90 satellites when intergrated with GPS systems from other nations. Couple that with advanced antenna technology from the receiver manufacturers and who knows. Until then keep the Agent Orange on hand and nearby.