GPS in wooded areas

There is an acronym for what you are proposing to do - GIS.

I've been using Topcon Hipers for static GPS for 10 years +. Here in Maine use in the woods is inevitable. When I need something fairly precise I make sure to stick with long observations -- 25 minutes at least and ideally 45 or more.Now and then more than an hour. You also ought to work with occupation planning software and know when the satellite configuration is best. Also look to minimize the canopy. Brush axe to clear out around the receivers. Base station is ideally in a wide-open area. We often have to set a station and backsight point in a more open area and GPS them and then shoot with T.S. from them in to the point of interest. There are definitely wooded areas where you're just not going to get usable GPS data. But when we've compared our solutions with lines shot with T.S. etc. results are generally sub-meter (which you want to build into your contract)(or often much better than that). I don't know anything about Trimble. My hunch is the same ideas apply. Someone on this site once wisely said "The great thing about GPS is that it always gives you a solution. The bad thing about GPS is that it always gives you a solution."

A WAAS enabled handheld will probably give you what you need. Then you won't be tempted to think it's better than it is.

I have a Trimble pro XRT with glonass that works great in the trees. It is a mapping grade unit so only sub meter but I have yet to be somewhere that it won't work.

Pine trees, or not? It makes a big difference.

Dry conditions are best.

Double your redundancy by having two units on control points in the wide open and a rover with an extra tall HI can increase you accuracy.

When the leaves are wet not much of anything helps.

😉

There is a way to do this...

For corner setting:
Think of a great tripod in the sky, with big holes in the brush to reach the sky. Think of using a float solution to get within 10' of the cor. Then, after getting that close, set the gps up, raise it up maybe 12 - 16 ft. Wait for a fix, and store it. Set a nail, and then inverse to the corner you need to set. Say, 276° 6.75' Use suunto declinated compass, and set temp cor. Move GPS to temp hole, and re observe.

OR if the cor is too near a tree, move to another location, and repeat, and reshoot in the corner with compass and tape. Compare the 2 locations. You must lose lock between GPS shots.

I typically get them within 0.10 or 0.15 ft on the first go at it, with an occasional bad localization, that has a 3-7 foot error.

The key here is DON'T get in a hurry. Take your time, and prove it.

N

R8-2s or R10s will work the best, but nothing will give you survey grade answers in thick canopy.

Javad Triumph LS. I'm doing the trial. Blows my R8-3 out of the water. You'll have to repeat your shots so you have some redundancy.

Javad. If they had marketing going right, and if they had TDS, then the game would be over.

Depends on your definition of "wooded area", and is "a few feet" = 3, 4, 5 ft?

No Garmin or other consumer GPS handheld will meet that accuracy, consistently, in the open sky, so under trees is out of the question.

Submeter receivers won't give you submeter results under tree canopy. Maybe 3 feet if the tree canopy isn't heavy.

> Any suggestions. Only looking for results within a few feet. Horizontal only required. Vertical not required. Is it possible to turn off the vertical solution with trimble for better /faster horizontal results? Does that even make any sense? What equipment/methods have you used and what were the results?

Someone has lead you astray if you believe that accuracy defines a GIS. There are GIS that require 1" accuracy and there are GIS that require 1000' accuracy.

> There is an acronym for what you are proposing to do - GIS.

The LS must be pretty good then! I have used R8-3 a lot, and they would do pretty good sometimes under canopy (not always).

Are you using VRS or the Triumph 2 base?

You can create a survey style that uses WAAS or SBAS in Access or Survey Controller. You'll see your "precisions" at the bottom of the screen just like normal GPS work. I've done this on a R8-2, a R6-2 and a Leica GS15. It worked for really rough locations and really really rough layout.

>turn off the vertical solution with trimble for better /faster horizontal results?

Re-read how GPS works. There isn't a separate height measurement. It measures times/distances from the satellites and turns that into coordinates. The measurement time affects accuracy. The calculation time to go from measurements to LLH should be negligible.

The poorer accuracy for vertical is because you have satellites in all horizontal directions (with any decent constellation) but only up and none down to help with the vertical. Thus propagation effects don't tend to average out like the partially do on horizontal, and are harder to distinguish from true vertical differences.

I am using VRS and the Triumph 2 base. The VRS network does not give me as reliable results as the T2 base. When I am using the T2 base I can get a repeatable (verifiable) result just about anywhere.

Matt

:good:

That is something I was wondering about the LS, does it have the dynamic glonass inter-channel bias calibration?

I figure JAVAD has just started putting it in all units?

It's not listed as an option for either the LS or the T2.

To be honest, I don't know. I have not heard anyone say whether it does or not.

What I can speak to is the ability to be sure that you are getting good repeatable shots even in tree cover.

The J-field program that is running on the LS and the Victor LS has a feature called verification. You tell it that you only accept fixed solutions, then you tell it you want to verify anywhere from 1 to 500 times with between 1 to 6 different rtk engines. Once you have a fixed solution with the number of rtk engines you specified, it will dump those fixed solutions and reacquire the fix. It then averages the fixed solutions out and give you an overall rms value and a peak to peak error that was seen over the course of the verification process.

This is a game changing feature for anyone who uses GPS. A way to be sure you do not have a bad fix!

Matt

The issue with GLONASS becomes significant if you mix base and rover. My understanding is that the Javad receivers treat each GLONASS signal uniquely (the wavelength of each GLONASS satellite signal varies, unlike GPS). This creates a unique situation with GLONASS. If I understand correctly, the interchannel bias is to correct signals from other reference stations.

If you are using the same brand of base and rover, generally it doesn't matter, as this bias exists in both base and rover and the errors generally cancel each other out.