# GPS a point under canopy

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### #1 Laura Miles Posted 31 March 2010 - 10:59 AM

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Hi Everyone, maybe someone can help me with this. I have a point that needs to be gps'd (or otherwise recorded), however it is under dense canopy so our gps unit can't get enough satellites. There is a road located about 20 feet from this point, so I was thinking I could gps two points on the road where there is no canopy, and measure the distance from each of these points to the point of interest. Then I could import the two points into ArcMap, buffer them by their respective measured distances from the point of interest, and one of the two intersections will be the location of the poi. I'm just wondering if anyone has another method or any suggestions, as I've never really done this before?
Laura

### #2 David Medeiros Posted 31 March 2010 - 11:07 AM

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Hi Everyone, maybe someone can help me with this. I have a point that needs to be gps'd (or otherwise recorded), however it is under dense canopy so our gps unit can't get enough satellites. There is a road located about 20 feet from this point, so I was thinking I could gps two points on the road where there is no canopy, and measure the distance from each of these points to the point of interest. Then I could import the two points into ArcMap, buffer them by their respective measured distances from the point of interest, and one of the two intersections will be the location of the poi. I'm just wondering if anyone has another method or any suggestions, as I've never really done this before?
Laura

Another option is to do what surveyors often do, coordinate geometry (I think thats it).

Record your benchmark location (the GPS point on the road). In your case I might record the same spot several times and average the points. Then take a compass bearing off that point to the point you want to record under the canopy. Walk it off with a tape and record the distance.

If the route to the hidden point undulates a lot is very far you may need to record intermediate distances. If it's up or down a steep slope you will need to record the slope to derive the point to point distance in plan view and not the walked distance.

Your version is simpler and in application essentially the same as doing it manually with a compass and tape. As long as you feel confidant about the accuracy and precision of your base POI on the road your method should work. Remember that the slope factor applies to either method. And keep the two recorded points as far from each other as possible.
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### #3 Laura Miles Posted 31 March 2010 - 11:41 AM

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Thanks for the advice David! One of our issues is we don't happen to have a compass around here, so I thought by taking the two points we could sort of solve that. It's going to be a challenge getting straight measurements as our POI is in the bushes!
Laura

### #4 David Medeiros Posted 31 March 2010 - 12:19 PM

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Thanks for the advice David! One of our issues is we don't happen to have a compass around here, so I thought by taking the two points we could sort of solve that. It's going to be a challenge getting straight measurements as our POI is in the bushes!
Laura

No compass?!

I think your method will work fine. My only concern would be the type and age of the GPS device your using. Is it a consumer grade handheld, or a Trimble unit? Will you be using differential correction? Anychance you have, or can get acces to a laser range finder?

Those things would help the accuracy but are not required of course. Straight lines between the base points and the POI are important though. How important I guess depends on the required accuracy of the project.
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### #5 Charles Syrett Posted 31 March 2010 - 12:28 PM

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I've done this kind of work before, and I would actually recommend that you combine methods. For a compass, use a "forester's compass", such as a Suunto. It's the kind with a mirror on it, available in any outdoors store. With this kind of compass, you can view along your tape and get the vertical angle -- handy for your later trig calculations. If you like math and outdoors work, this should be fun!

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### #6 Nick H Posted 31 March 2010 - 01:17 PM

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Thanks for the advice David! One of our issues is we don't happen to have a compass around here, so I thought by taking the two points we could sort of solve that. It's going to be a challenge getting straight measurements as our POI is in the bushes!
Laura

I shouldn't be too concerned about the lack of a compass. If you have accurate coordinates for the two fixed points it's easy enough to calculate the distance between them (this will be your baseline). Then you can measure the distance from each end of the baseline to the feature you are surveying. You'll then have all three sides of a triangle, which is all you need. Surveying by taking measurements from two points of known position is very accurate. It might be also be a good idea to measure the length of the baseline if possible and compare it with the calculated value.

Gonzo surveying! I love it .

Regards, N.

### #7 Charles Syrett Posted 31 March 2010 - 02:02 PM

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The advantage of the compass is in correcting the taped distance measurement. Are you capable of holding a tape in a perfectly horizontal position, in the middle of a dense forest? Me neither. You'll need that compass to record the vertical angle of the tape, which you will then use to correct the measurement you took.

Of course, if your change of elevation is negligible, then your distance measurement probably will be as well, and you won't need a compass.

Charles Syrett
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I shouldn't be too concerned about the lack of a compass. If you have accurate coordinates for the two fixed points it's easy enough to calculate the distance between them (this will be your baseline). Then you can measure the distance from each end of the baseline to the feature you are surveying. You'll then have all three sides of a triangle, which is all you need. Surveying by taking measurements from two points of known position is very accurate. It might be also be a good idea to measure the length of the baseline if possible and compare it with the calculated value.

Gonzo surveying! I love it .

Regards, N.

### #8 Laura Miles Posted 31 March 2010 - 02:12 PM

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I do love math...
A laser range finder...only in my dreams. We can't even manage to obtain a compass. I will be using a Trimble hand-held unit, it can get up to 30cm accuracy. Just curious if the 'pros' out there thought this method would yield an accurate location, sounds like it should.
Charles, what you describe is how I learned to map in my very first Intro to Cartography class. (Memories!) I figured with the absence of a compass, but the use of a GIS, I could determine the POI accurately enough. (The elevation change is negligible in this case. Not sure yet how much of a problem the forest will be though.)
Thanks all for the expert advice!
Laura

### #9 David Medeiros Posted 31 March 2010 - 02:25 PM

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If you find a tape unwieldy in the bush (even a proper survey tape) you can use a low stretch poly or nylon line between the two points. Pull it tight and mark both ends. Take it out to the road, pull tight again and measure.
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### #10 Nick H Posted 31 March 2010 - 02:32 PM

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The advantage of the compass is in correcting the taped distance measurement. Are you capable of holding a tape in a perfectly horizontal position, in the middle of a dense forest? Me neither. You'll need that compass to record the vertical angle of the tape, which you will then use to correct the measurement you took.

You're right here of course Charles, if there's a slope this has to be taken into account.

If you could see GPS Point 1 from GPS Point 2 another way of doing the job would be by offsets. Measure the distance from the feature under survey to the nearest point on the baseline and then measure the distance from that point to one of the ends of the baseline. You'd need three rods and a chain-boy for this exercise, but Gonzo surveying by offsets can be very accurate too.

Regards, N.

### #11 frax Posted 01 April 2010 - 04:13 AM

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It is not that the GPS unit has a digital compass... ?
Hugo Ahlenius
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### #12 Laura Miles Posted 01 April 2010 - 09:18 AM

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