I'd like to get some feedback about a GPS receiver's electronic compass.

Do you think they are accurate?

What will interfere with them?

Are they worth the extra cost?

Do they use up too much batter power?

Thank you,

Blake

Considering how easy it is to navigate with a conventional map and compass, I would not pay any significant amount for a GPS electronic compass. I do find a GPS an invaluable tool, but its Achilles heel is the finite amount of battery power. I would not care to squander that juice on a compass function.

My very basic GPS receivers will give me a direction if I move just a short distance (25 feet or so) if I must use my GPS in order to get a bearing, I can do so.

I have an electronic compass incorporated into my wrist watch, and it is sort of comforting to have it there as a backup, but I don't think I have ever had to use it for real.

In the fairly mountainous, feature laden terrain common in most of the western US, one rarely needs to use any compass. Orientation by terrain features is quick and at least just as accurate as using a compass. Get out on the water and bring on a nice pea soup fog, or trek to the north woods, and it is a different story.

GPS doesn't know what direction it is pointing when it is not moving, or is moving very slowly. Thus a supplemental compass might prove useful.

But a magnetic compass is so cheap and simple that the absence of a compass function in an outdoors oriented GPS is not much of a deficiency.

Most of my GPS units don't have built-in compasses and I learned my GPS usage that way.

In my GPS receivers that have a compass I have turned the electronic compass Off. It uses battery power better conserved for the receiver. As was stated, a simple magnetic compass works without using any battery power.

Concur with using a Magnetic Compass. I carry the Silva Ranger in my pack and a Brunton baseplate in my go bag.

Good, electronic compass battery drain is significant.

Baro altimeters OTOH are useful. With the GPS elevation updating baro altimeter, you end up with a fairly accurate vertical measurement. The baro alt doesn't drain the battery.

I have seen conflicting comments about the utility of "GPS altimeters" - those fixing the altitude solely by GPS geometry and not utilizing barometric correction. While some claim that this is quite inaccurate, I have found that my bare bones receiver, a Garmin Foretrex 101, is actually quite accurate.

Due to some recent street work, I know that my driveway slab is 201 feet ASL. I usually use my Garmin to track my mileage when I leave the house on a bike ride. Often the displayed altitude is within 0 to five feet of the actual elevation, if I have turned it on and given it time to settle down and figure out where it is. A lot of my riding is along the coast and I can eyeball my altitude and compare it with the GPS determination. Again, it is usually good enough for all practical purposes.

There are exceptions - I have been told I was 48 feet below sea level when I was clearly at least fifteen feet above the water, so there can be hiccups (satellite configuration?) in the system, but they generally resolve in a minute or two.

I remember an instance when I was slogging up a long grade on my bike in South Dakota. The altimeter was very useful in giving me a useful representation of how much I had gained, and how much was left. The true altitude could have been off by as much as 100 feet or so, as far as I know, but the computed altitude was still good to know.

I believe generally speaking a barometric altimeter (calibrated occaisionally to a known altitude point) will be more acurate than an altitude from GPS coordinates. As I understand it, the issues are as follows:

Correct altitude is measured from the "Geoid" which is the actual sea level gravitational potential surface. Due to variations in density in the earths crust, the geoid is a rather "lumpy" and irregular surface, and difficult to handle mathematically. GPS coordiates are measured relative to a "datum" (WGS84, NAD27, etc), which uses a mathematically defined "spheroid". This speheroid is mathematical approximation of the geoid. How accurate your GPS (calculated from satellites) elevations are depends on how closely the spheroid matches the actual geoid at that particular location. Thus, a GPS derived elevation may be spot on....or may be as much as 100 ft (30 m) off.

Some GPS units include a barometric altimeter, which measures altitude from air pressure. Of course air pressure varies with weather changes. Hence the barometric altimeter must be calibrated periodically at a known eleveation. Bench marks, spot elevations such as mountain tops, etc are good places to calibrate your altimeter. Map contours are subject to their own errors, so while still useful, they are not quite as good for calibrating an altimeter. In general a barometric altimeter, calibrated now and then, will be more accurate than an elevation derived from GPS satellites.

Following up on my ealier post regarding GPS alitmeters vs barometric altimeters. I'm a bit of a map geek, and to some extent use this stuff in my work, but the whole concept of geoids, spheroids and datums gets very confusing to many people. If you want a little more understanding of these subjects, I can recommend two sources:

The book How To Read A Nautical Chart by Nigel Calder (2003, International Marine) has by far the clearest discussion I have seen for explaining the geoid, spheroids and datums, and why they matter. The book is aimed at sailors, but the first couple of chapters are useful background for anyone using maps and charts of any kind.

Another source, which is the book Geodesy For The Layman (1984) which is available online from NOAA at
http://www.ngs.noaa.gov/PUBS_LIB/Geodesy4Layman/TR80003A.HTM
and in pdf at
http://www.ngs.noaa.gov/PUBS_LIB/Geodesy4Layman/geo4lay.pdf

My watch also says to recalibrate the compass each time in a new location, but I don't and it still points to north. Might be a few degrees off, but doesn't affect anything.