Dear list,
I received a lot of great hints for marking a grid in the field using GPS
methods. Thanks to everyone who replied off-list!! A few people asked me to
share the replies, so I thought I'd compile them and post them here for all to
see.
Thanks again, all.
Lauren Quinn
Sent:
Fri 6/18/10 6:54 PM
To:
lqu...@hotmail.com
You can probably create a track by walking the grid once, save it,
and then reload it to use it as a template for where to walk again.
Sent:
Fri 6/18/10 7:00 PM
To:
L Quinn (lqu...@hotmail.com)
Hi Lauren,
One
thing to consider is the accuracy of the gps unit. The positional error
associated with a garmin can be a couple meters, especially if you are
moving around. If you are working on a fine scale you may want to stick
with measuring tapes. Good luck on your project.
Sent:
Fri 6/18/10 7:30 PM
To:
lqu...@hotmail.com
Hello,
>From my limited use of GPS units, I do not believe that
you will achieve anywhere near the accuracy you desire, using a handheld
Etrex. There are several factors to consider, and you would probably
find the GPS solutions either too cumbersome or too expensive. I will
leave it to the experts to advise you further on the model you would
need to achieve that accuracy.
You would probably do better with
getting good maps for wherever you go, and then using a compass
(compensate for magnetic versus true north) and your GPS to navigate the
area. If you are concerned about covering the same ground twice by
mistake, you can use a marking system to show where you have been (we
used toilet paper draped on low bushes in Minnesota. It's biodegradable
and will disappear the first time it rains). I would concentrate more on
the goal of the sampling system (achieve separation between
samples/cover the entire area) rather than focusing on whether I was in
exactly the right grid spot (unless that is the essential goal, hard to
say without knowing more details).
One final tip. If you know the
spacing, instead of using a measuring tape it may be quicker to
pre-measure a length of rope (one that doesn't stretch or shrink when it
gets wet) and use that in the field. Works better when you have a
person on either end of the rope.
Also, consider an infra-red range
finder.
Test your GPS in an open sports field. Walk the marked
lines and see how accurate the resulting GPS track is. That will give
you a rough idea of the accuracy you can expect.
Good luck!
Sent:
Fri 6/18/10 7:35 PM
To:
lqu...@hotmail.com
Cc:
ecolo...@listserv.umd.ed
Lauren,
It is possible to do what you are asking, but it is not easy. It
would be easier to arrive at your starting location, project one
waypoint in the direction you want to go (along a desired grid line), do
a "go to" to the projected point, and walk offsets of 0m, 20, 40, etc.
from the "go to" line. I suggest you read your garmin manual about these
topics, as I could not explain all of this from scratch to a
5-year-old.
Sent:
Fri 6/18/10 7:48 PM
To:
L Quinn (lqu...@hotmail.com)
Dear Lauren,
I used to use a similar method for generating transects for surveying
birds with an eTrex Legend. I used Garmin's MapSource software (which
is simple and inexpensive) to program routes and then would sync them
with the GPS. You could create a grid simply using a series of
intersecting, straight-line routes. I would also use a feature in the
GPS (I think the eTrex can do this but I can't remember for sure) called
"project waypoint." After marking a waypoint, you can use the editing
tool for the waypoint and move the point by designating a starting
point, a distance, and a bearing. This was an easy way of creating
transects on the fly. Projecting a waypoint can be done anywhere as
long as you have a reference point for your field site. I'm not sure if
that is clear or helpful or quite what you need. If you would like
further details or help, please let me know.
Sent:
Fri 6/18/10 8:10 PM
To:
L Quinn (lqu...@hotmail.com)
Hi Lauren,
You can load lines onto your Garmin
unit as tracks -- the best way to do it is using a free program called
DNR Garmin created by Minnesota DNR (can download online). If you
can use ArcGIS to generate a fishnet grid (XTools does this easily)
shapefile
for your study area, you can bring this into DNRGarmin, then save it as
a track file with a .gpx extension. This
