Subject: GPS Usage by Birders??
Date: Jan 18 15:05:05 1998
From: DAVID NUNNALLEE - davidn at nwlink.com


Deb Beutler wrote:

> I use a GPS for my thesis research project on cowbirds. I use the
>GPS to determine the location of my study sites on the earth so that I can
>map them on a Geographic Information System (GIS) computer database.

GPS is an excellent method for recording localities of any kind. This method has been in use for several years for recording dragonfly and butterfly localities, for example. One big advantage over recording a locality in the traditional "find-where-I-am-on-the-map" method is that you don't have to depend on the accuracy of incomplete maps, and you don't have to search for those mapped roads which no longer exist.

> I will confess, I have never used one to navigate through an area.
>I use landmarks to navigate and get lost if I try to navigate using a
>compass and map (or GPS).

I think the navigation-by-GPS features are largely overstated. It can be done of course, but most of use don't really need this feature. I have occasionally used my GPS to simply get a solid compass direction from my present location to my starting point (ie, my car), which can be valuable in confusing country. A compass is also needed to translate this information into usable directions, of course.

>Sometimes I look at a map and I am amazed at how
>close some of my sites are "as the raven flies"; it takes a long time to
>drive between them!!

Amen to that. I have also been impressed in this way.

> One word of warning for those of you depending on GPS to navigate:
>they don't always work well. In northeastern Washington, I often have
>trouble locking on four satellites (the minimum for my system), even in
>relatively flat terrain, particularly during the early mornings. They can
>be blocked by mountains very easily. Also, they may not work dense forests.
>We can't get a satellite lock with our GPS if the canopy is even relatively
>closed. I assume similar problems would arise in old-growth forests on the
>west side.

For whatever reasons, I have occasionally had trouble locking on to satellites even in the wide-open prairie country of eastern Washington. The first thing I always check is my batteries; my GPS unit starts responing poorly (or slowly) whenever the batteries become weak, even though there has been no low battery warning. Like any battery-operated unit, cold temperatures can reduce the GPS performance significantly too.

One key thing to keep in mind, as Deb mentioned, is blockages. Hills, man-made obstructions and trees do indeed block GPS signals. But keep in mind that GPS units use two different kinds of antennae, and the difference is rather important. Those units with no visible antenna usually have a horizontal built-in antenna (as in the Magellan units). This kind of antenna tends to "see" the sky directly overhead quite well, and thus should be less influenced by obstructions on the horizon. The other type of antenna is typically a flip-up type (as in my Garmin GPS) which points vertically when in use. This type "sees" the horizon very well, and is best at finding more satellites, but it is also more easily blocked by obstructions on the horizon.

>The system we have
>can't determine elevation with any accuracy. There have been many times
>that I have walked up hill for several meters and had the elevation reading
>go down!

As others have indicated, the elevation feature of GPS units is virtually useless. The best advice is don't even try.

Others have commented on the military's built-in error feature, which presumably makes Sadam's atomic bombs so inaccurate that they will hit next door rather than directly on top of our house. I have heard that the military intends to remove this built-in error feature from their GPS signals, in which case the little hand-held units will become much more accurate. At that time (maybe a couple of years hence?) we can reevaluate the elevation feature, and see whether it is usable then.

>The system I use can't be used in a vehicle; I have to stop and
>get out to get a position reading.

If your vehicle has a somewhat sloping windshield, the GPS unit can be placed on the dash and will actually work through the glass. Many brands also have exterior plug-in antennas available (at a significant cost) which can be placed on top of the car. One nice thing about using the GPS in the car is that you can use a cigarette lighter power adapter and not run down your GPS batteries. (Oh BTW, when buying a GPS unit, take careful note of battery life. It varies *dramatically* from brand to brand!)

Oh, one other thing. Once you have your GPS localities all written down and recorded, you will find that plotting such localities on the map is a major pain. Unfortunately, few maps have latitude-longitude grids; take a look at the Delorme maps, for example. You will find only occasional little tick marks every 7.5 minutes along the margins. You can plot your GPS locality on such a map with considerably accuracy, but it requires a lot of interpollation and patience. Hopefully the next generation of DeLorme maps will include "lat-long" lines!

Many GPS makers also sell download module accessories, some at a reasonable price. These units attach your GPS directly to your computer, and with software provided with the unit this allows your GPS readings to be sent directly into a database. There is also mapping software on the market (I haven't actually seen it used) which will automatically plot your GPS readings on a base map.

Dave Nunnallee
davidn at nwlink.com
Issaquah, WA