[permaculture] GPS Experience in PC

darren darren at permaculture.biz
Sun Apr 3 14:18:04 EDT 2005

Hello People, 

I trust that this greets you and all well. 

Scott Pittman privately prompted the following addition to my last rave: 

I neglected to mention my experiences with GPS systems. I have a Garmin 
Etrex Vista C which is a very useful tool. However its positional accuracy 
(like most consumable GPS's) is at best around 3m. The internal barometric 
altimeter, as with most GPS's is around double that. I use a surveyor at 
home who uses both a total station (remote one-man) and a Trimble DGPS 
(Differential GPS) instrument that is about 1-10cm accurate and double that 
for elevation. The differential GPS differs to the normal units in that it 
picks up on signals from terrestrial beacons. This is very cool though the 
setups are minimum AUD$2-5K+++ and you have to subscribe to the signal 
provider for access. 

These setups are roughly analogous to systems used in broadacre cropping 
("tramlining" and self-steering tractor setups), which are generally, in 
Australia at least, round 2-20cm accurate. These systems are used for 
sowing, fertiliser and biocide application, yield monitoring, and typically 
have all farm machinery movement follow a single path - reducing compaction. 
That's ok in theory though most machinery has a different width track - 
except on those "perfect farms" with an overdraft matching some countries 
entire GDP. 

My experience with my handheld GPS has been relatively positive. It is great 
for taking track logs and for doing area statements - even monitors ground 
speed and distance travelled at the same time as taking elevational data. So 
for example when I was keyline plowing a client's paddock I had the gps 
recording the speed, distance and elevation. I then plugged the info into 
the notebook, then put the data into MS Excel as a .txt file, cut it down to 
just X,Y and Z (longitude, latitude and elevation respectively) data and 
brought it into AutoCAD Land Desktop (which I trialled as a 30 day bomb at 
the time). From this I made a contour plan. Sounds good so far. 

At different times of the day there are different numbers of satellites that 
your GPS will be receiving. The more satellites the more accurate its data 
will be. The same appears to apply for barometric readings. So cutting to 
the chase the resulting contour plan is about a 50% result - with high 
relief rises and troughs appearing all over the place. You can clean this up 
of course but the vital question is what is the accurate data? Similarly I 
have had the same experience using the GPS to measure distances - sometimes 
its good sometimes not so good. 

So after putting my Garmin through the "ringer" I have concluded that it is 
best used during site analysis for marking basic positions of points of 
interest and for fences, roads, remnant veg etc. Things that do not require 
highly accurate positioning. Also for noting tracks etc. That has been 
valuable over here in Viet Nam so I can remember (the internal memory card 
that is) how to get back to somewhere etc. 

Otherwise the DGPS is another matter - I could get into that though I think 
my time is better served to do things other than surveying. 


Yours and Growing, 

Darren J. Doherty 

Integrated Consultant/Lecturer
M&M's/Mars Inc., ACDI/VOCA
Sustainable Cacao Agroforestry Systems (SCAS) Project,
Forest Science Institute of Vietnam (FSIV) Research Farm, Huong-lo 312, 
Nghia Trung Commune, Bu Dang District, Binh Phuoc Province, Socialist 
Republic of Vietnam 

m. +61 (0) 418 254 605
e. darren at permaculture.biz
w. www.permaculture.biz 

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