GPS'ing for elevation

[argohauler]

I was using my Garmin etrex Legend H for some elevation work in the bush. I wanted to know how much fall there was between the start and end of my mainline. So I punched in the end of the mainline. I then went up the hill directly to the north of the mainline end and it was a lower elevation then at the bottom of the crick. Went back the next week stood in the same spot at the end of the mainline and it was a totally different number.

Do I need a better GPS? Would we be using the same satellites in Canada as the US?

[GGervais]

We use the same satellites. I am a licensed surveyor and have a lot of experience with gps. The signals you recieve from the satellites through the canopy are not as good as in an open sky leaving you with bad data. the more satellites you can lock onto the better your position usually is. Also a handheld gps unit is only accurate to within 3 meters with open sky so your readings will probably be different with every reading. If you use google earth it will give you elevations that may be better than the ones from your gps.

[PerryW]

Yes, GPS is very popular for surveying nowadays, but us brush apes here in the northeast have that pesky canopy of trees. One of the surveyors I work for, has survey-grade GPS receivers (Promarks, I think) but we really don't use them that much, unless it's a clearcut or open fields.

Surveying in this area is pretty much still about chopping line and running a conventional traverse with a Laser Theodilite (Total Station) to get the necessary. Just chopped out 10,000' of line myself last monday for a land survey.

GGervais, Check out www.Beerleg.com if you like talking surveying.

[mapleack]

Usually the vertical or elevation margin of error is three times that of horizontal accuracy. So if your handheld gps is accurate to 10 feet horizontal, it would only be accurate to 30 feet vertically. Tree canopy makes it impossible to get survey grade gps posistions. It's possible in the winter, but still more difficult than out in open ground. To get accurate elevations with gps you're talking about a 20k plus piece of equipment. I have access to all the fancy stuff, but still use a tripod mounted construction level to check low grade mainlines because it's quicker. I start at the bottom, painting a level line on each tree the mainline touches, moving up in stages as necessary, then you can use a tape measure and the level lines to grade the mainline. Every time you have to move the level you put two lines on the overlapping tree.

[Newfvt]

I guess I am glad to see these posts because I have had exactly the same unfortunate experience.

6 years ago I opened about 30 acres of new sugarbush and had the bright idea of using GPS to set mainlines. Since we are on environmental center land we had access to donated state and utility company GPS units and techs as "free resources". A GPS instructor for a local college wanted to develop a GPS map overlay that would allow one to take a cross section through any part of the sugarbush and visualize tree locations and slope/grade along a run of tubing or mainline. If the idea had worked, we would have had a software program to give away to other sugarmakers. Way cool!

We numbered every tree. The GPS techs could tell you within a few inches where a given tree was on the surface. They could tell you very accurate distances between trees. But as has been said here, when it came to elevation, the data was all over the map.

This work was done early in spring with a very light bud on the trees - sky was very visible. We actually set a main line using data we thought was reliable. It was along the edge of an open field, but under the canopy of the trees. On a run of about 550 feet the GPS elevations were off by over 2 feet on the bottom end which was about 200 feet across a hay field to a road side tank location. . To make it work, the last 100 feet of line was almost touching the ground and we had to set the tank down into the soil by at least 2 feet.

Maybe someday!

Steve

[PerryW]

yup, Land Surveyors could talk all day about the problems that have been created by the misuse of GPS. At least you don't have to tear down a house because it's over the line.

[adk1]

Your best bet is to just use a USGS topo map, can be obtained in many places. They are based on 20' contours but better than a standard gps for sure especially in a canapy

[Sunday Rock Maple]

We just did 22,000 of main line this year using an old 3 post transit with a site level --- not as high tech but worked well.

[argohauler]

We used a transit originally 11 years ago.

Thanks for the replies.

[The Sweet Spot]

If the GPS can accurately map the trees, is there a way of overlaying the tree locations on a USGS map?

[Maple Hunter]

If the GPS can accurately map the trees, is there a way of overlaying the tree locations on a USGS map?

Yes, you can do that with nearly any basic Topo software and easily with ArcMap (ArcGIS). However, I wouldn't trust the contour intervals completely, as they're not always perfect either. Has anyone tried an altimeter? Some are fairly accurate. I know one of my GPS units has one built in, but I've never messed with it much to know it's accuracy.

[The Sweet Spot]

Thanks Maple Hunter, I'm not the only one to notice this. The usgs shows my lines very different than they are.

[theschwarz1]

Many states and/or counties have flown LiDAR and have generated very accurate contours and hillshades. Look up your state or county's GIS daa. If you want free GIS Software I can help you with that.

[theschwarz1]

If the GPS can accurately map the trees, is there a way of overlaying the tree locations on a USGS map?
YEs there is. If you want I can help you create a very nice map with your GPS points and showing accurate elevation.

[maple flats]

I believe topo maps are drawn from areal photos. As such, lines are drawn thru a picture full of trees. Can't be 100%, but the get fairly close.

[maple man]

Best way to get most out of your elevation is run your wire. Then start at the releaser go to first tree it touches we made a two foot level with hook on one end .and 2/3/4/5 % hookes on the other end . Then work your way up the line in betewn tree start at 2% if geting to low go back and start over. When you have set right put nail in tree to hold wire and works as good referance point for future if tree takes line down. I do now side tying . If tree falls on line I can have back up level in now time.