GPS & Survey
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GPS & Survey
What experiences ahave people had of using GPS as a survey tool in wooded terrain ? Are they worth the investment, and if so any recommendations about whats good, what to look for, what to avoid.
- red adder
Last map I did I used a Garmin eTrex Summit (which includes alitmeter). It was useful as base information alongside the OS and (in this case) the previous o map. In a lot of ways the altimeter was more useful when trying to work out which contour to drag in to make a particular feature.
I couldn't recommend using it for absolute point mapping though because it isn't accurate enough. Yes it can be accurate down to (reported) 4 metres but that's either way so as it jumps between satelites it jumps 4 metres one way to 4 metres the other - 8m out. In the woods the signal wasn't dropped too badly but it was degraded a bit to a reported 6 or 7 metres. Sometimes accuracy drops right down to 20m but that didn't seem to be related to woods or open but more the path of the satelites I guess.
I couldn't recommend using it for absolute point mapping though because it isn't accurate enough. Yes it can be accurate down to (reported) 4 metres but that's either way so as it jumps between satelites it jumps 4 metres one way to 4 metres the other - 8m out. In the woods the signal wasn't dropped too badly but it was degraded a bit to a reported 6 or 7 metres. Sometimes accuracy drops right down to 20m but that didn't seem to be related to woods or open but more the path of the satelites I guess.
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FatBoy - addict
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There has been some discussion of this on the O-Map group in Yahoo groups... I attached a quote here... this is a post by Ken Dowling and refers to consumer grade units
QUOTE>>>>
Recently I used GPS extensively to map a track network for a mountain
bike enduro navigation event. This event did not require orienteering
level detail and the GPS quality was fine (provided I paid careful
attention to the way in which I used it and how I applied the
subsequent track plots).
Along the way I came to some conclusions on useful functionality for a
GPS unit.
1. Must have: A multi track or multi segment track function would be
top of my wish list. My Magellan Meridian records just one track and
interpreting multi segments and back tracks from that one track was
tiresome and a bit error prone.
2. Highly desirable: 8 hours worth of trackpoints - my Meridian can do
2000 trackpoints. My settings are for finest detail. 2000 seems enough
for at least 4 hours of winding tracks but a couple of times I had to
cut short a longer day.
3. Essential if #2 not provided, else desirable: A display of track
points used/remaining.
4. Must have: Track Glonass satellites - my Meridian doesn't.
5. Desirable: Ability to create own symbol set for waypoints.
6. Useful: And a compass function that works while standing still.
===============================
A tool that I cam across too late to get enough benefit from for that
map is the Trimble Planning Software (free from http://www.trimble.com)
This software shows the spread and angle of satellites for any
location for any time span and calculates various useful data. You can
even set parameters to signify horizon obstructions (e.g. cliff,
mountain...)
I didn't think much of it on initial checkout. But a few days later I
fired it up and was astonished at the superb DOP (dilution of
precision) readings for my area for half the day. I got out there and
sure enough my GPS was receiving multiple strong signals throughout
that period even under tree cover.
So if you have the luxury of choosing when you go out GPSing I
strongly recommend the use of the DOP function of this utility. You
can plan as far ahead as you like.
Thanks Trimble.
Ken Dowling, Australia
<<< END QUOTE
My own experience is limited but I do know that you shouldn't trust a consumer unit when moving around near trees! Specifically the units tend to assume you are travelling in at constant velocity and direction when they lose sight of the satellites and just propagate your position until the lock comes back.
Given a reasonable view of the sky and more advanced receivers (differential) you can obtain very high level accuracy but then perhaps you could see that reference point from an stereo aerial photo anyway?
and off subject... My favourite gadget is a LASER range finder... e.g. Bushnell Yardage Pro Trophy Laser Rangefinder... bought it ages ago and have never got to use it for a map yet but good fun!
Regards,
Guest
QUOTE>>>>
Recently I used GPS extensively to map a track network for a mountain
bike enduro navigation event. This event did not require orienteering
level detail and the GPS quality was fine (provided I paid careful
attention to the way in which I used it and how I applied the
subsequent track plots).
Along the way I came to some conclusions on useful functionality for a
GPS unit.
1. Must have: A multi track or multi segment track function would be
top of my wish list. My Magellan Meridian records just one track and
interpreting multi segments and back tracks from that one track was
tiresome and a bit error prone.
2. Highly desirable: 8 hours worth of trackpoints - my Meridian can do
2000 trackpoints. My settings are for finest detail. 2000 seems enough
for at least 4 hours of winding tracks but a couple of times I had to
cut short a longer day.
3. Essential if #2 not provided, else desirable: A display of track
points used/remaining.
4. Must have: Track Glonass satellites - my Meridian doesn't.
5. Desirable: Ability to create own symbol set for waypoints.
6. Useful: And a compass function that works while standing still.
===============================
A tool that I cam across too late to get enough benefit from for that
map is the Trimble Planning Software (free from http://www.trimble.com)
This software shows the spread and angle of satellites for any
location for any time span and calculates various useful data. You can
even set parameters to signify horizon obstructions (e.g. cliff,
mountain...)
I didn't think much of it on initial checkout. But a few days later I
fired it up and was astonished at the superb DOP (dilution of
precision) readings for my area for half the day. I got out there and
sure enough my GPS was receiving multiple strong signals throughout
that period even under tree cover.
So if you have the luxury of choosing when you go out GPSing I
strongly recommend the use of the DOP function of this utility. You
can plan as far ahead as you like.
Thanks Trimble.
Ken Dowling, Australia
<<< END QUOTE
My own experience is limited but I do know that you shouldn't trust a consumer unit when moving around near trees! Specifically the units tend to assume you are travelling in at constant velocity and direction when they lose sight of the satellites and just propagate your position until the lock comes back.
Given a reasonable view of the sky and more advanced receivers (differential) you can obtain very high level accuracy but then perhaps you could see that reference point from an stereo aerial photo anyway?
and off subject... My favourite gadget is a LASER range finder... e.g. Bushnell Yardage Pro Trophy Laser Rangefinder... bought it ages ago and have never got to use it for a map yet but good fun!
Regards,
Guest
- SLOW Foxtrot
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