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| Subject: | GISList: [SUM2\ GPS in thick forests |
| Date: |
05/06/2002 09:53:04 AM |
| From: |
Xiaohua Dai |
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Dear Listers,
There are some more good suggestions for my original question about the imp=
roving of GPS accuracy under dense canopy:
------------------------------------
1 Dr. Duane F. Marble:
Working in Costa Rica, there are no gaps at close enough intervals to permi=
t this. Also, at La Selva, you are not permitted to cut vegetation that wou=
ld get in the way of the laser beam. (cv. 1st point of my last SUM)
A major pain to lug around and then with a high canopy it does not reach mo=
st of the time. The canopy can also be very difficult to penetrate. (cv. 2=
nd point of my last SUM)
We ended up doing a ground survey with the surveyors having to tie limbs ba=
ck, etc., in order to clear the line of sight without destroying the vegeta=
tion. Permanent markers (several thousand of them) were then set on a 50 x=
100 meter grid. Measurements are then referenced to these. (This is a ver=
y good idea!)
----------------------------------------
2 Gaster Kawuubye Kiyingi:
Try using a Differential GPS. I Know company that manufactures them in South
Africa and Spain.
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3. David La Puma:
I found that the Trimble ProXRS with the TSC1 worked very well in dense for=
est cover in Costa Rica. One of the reasons was probably the fact that we w=
ere working on slopes that allowed for a "sprinkle" of sky view most of the=
time...but it was
minimal. I think the days of raising your GPS antenna above the canopyare n=
early over, as we didn't usually raise ours over 5 meters (and often only 2=
-3) the whole trip. Trimble offers the ability to switch between 2-D and 3-=
D data collection on the fly, which is great when the conditions are less t=
han optimal but your elevation is relatively constant. The only problem tha=
t we had was no satellite service for real-time data correction, and no bas=
e station within a reasonable
distance . If we had to do it again, I would set up a second ProXRS as a ba=
se station and would be able to correct from 1 - .10 meters accuracy (right=
now it's 5 - 50 meters). The alternative is to get a satellite subscriptio=
n, or if in the United States, use the radio beacon option (this works grea=
t here!).
---------------------------------------
4. Jesse Mohr :
If possible plan your data collection when the surrounding vegetation has t=
he least amount of leaves. In the pacific northwest of our forests are con=
ifer dominated but much of the middle story shrub and small tree species ar=
e deciduous and lose there leaves, greatly increasing sight and GPS reading=
s.
-----------------------------------------
5. Martin B. Jones :
We use the OmniSTAR DGPS with a laser rangefinder device called the Criteri=
on 400 for our mapping work. Apart from being able to measure tree heights,=
diameters and basal area, the Criterion allows us to shoot positions when =
the canopy is too dense for the DGPS. We take our position outside the cano=
py using the DGPS and then shoot and map positions using the laser.
------------------------------------------
Thanks for their kindly advice!
Xiaohua Dai (Mr.)
Institute of Botany, School of Life Sciences,
Zhongshan University, Guangzhou, Guangdong Province
P.R.China
Zip Code: 510275
Tel: 0086-20-84111117
Email: dxhchina@sina.com
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0=B2=A9=CA=BF
=D3=CA=B1=E0=A3=BA510275
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