>I'll be interested in seeing if there's any response to this on the list. My >views of GPS on the canals are well know, but as keen hill walkers Mrs Steve >and I have been thinking of investing in one for our trekking trips. But If >at best they're 500 ft out of kilter (and that's before degrading) it hardly >seems worth buying one. We can read our OS maps wrongly ourselves without >technological assistance, thank you very much.
Here is my limited understanding of GPS. - Selective Availability was turned off in 2000, and as far as I know there are no plans to ever turn it on again (especially considering how much GPS is being used in civilian life). - GPS accuracy depends on a lot of variables. One of the variables are the satellites themselves. At a minimum you need 3 satellites to calculate a 2D position (4 satellites for a 3D position). But one of the factors is the position of the satellites in the sky; a clump of satllites close together is less accurate than satellites spread over the sky. Additional satellites improve accuracy (but there is a practical limit, of course). I generally see 6-8 satellites if my GPS has had enough time to find them. Many GPSes have an "accuracy" display on them in units of feet/meters. This is (in most cases) a variable called the Estimated Position Error. What this means depends on the GPS manufacturer, but in general it's a confidence circle ... some percentage of the time your fixes will be within the circle given by the EPE figure and the rest of the time they won't; the percentage I've seen is anywhere from 68% to 95%. So if you have an EPE of 15 feet (for example), 95% of the time your position should be within 15 feet of the displayed position. - One thing many people don't consider is a factor called the "datum". The datum is a mathematical model of the earth's shape (in reality the earth doesn't meet any of these datums). The latitude and longitude lines are referenced to this datum. GPS uses a datum called WGS84, which is very closed to a datum called NAD83 (I don't remember what WGS stands for, but NAD is "North American Datum", and it was developed in 1983). On good maps the datum is almost always printed somewhere (usually in very tiny type). It kinda surprised me when I found out that there are a LOT of datums; many countries have created a local datum that makes their part of the earth relatively uniforum at the expense of making the rest of the world not quite as uniform. Unfortunately difference in position given by two datums can be rather large, and the amount of difference depends on not only the datums in question but your position on the earth. Here is an excellent image which shows the position shifts seen by various datums at one particular spot (the Texas State Capitol Dome). http://www.colorado.edu/geography/gcraft/notes/datum/gif/shift.gif Note that some datums have position shifts from WGS84 exceeding 500 meters. Some GPSes have the ability to use different datums (my handheld Garmin GPS has this capability). In my personal experience, GPS is very accurate given clear access to enough satellites. I generally see accurates in the 10-15 feet range. If you have a good lock but are seeing inaccuracies in the hundreds of feet or meters, that sounds like a classic problem of a mismatched datum. I believe that the British Ordnance Survey maps use their own datum, so if I wanted to use a BOS map with my GPS I'd make sure I could change the datum on my GPS (I'd have to look, but I know my handheld Garim has a ton of additional datums so it would surprise me if it didn't have the Ordnance Survey Datum). One additional note: >As for the powers that be they feel (it seems) that the civilian GPS >needs to be reasonable accurate to a hundred yards or so. For, as a >pilot pointed out, as long as the ones used by civilian airliners can >get them to the right airport that is all that's needed. You may not be aware of this, but the US Federal Aviation Administration is massively pushing the use of GPS to use for instrument approaches (non-visual landings done in bad weather). Many airports have GPS approaches now. A hundred yards left or right of a runway could definately have hazardous consequences. There is an additional signal called WAAS (Wide Area Augmentation System), but the primary purpose of that is to increase the vertical accuracy of GPS and correct for a number of atmospheric errors. As I understand it, the FAA was one of the major advocates for the removal of SA from GPS. This allows the FAA to eventually decommission a large number of expensive-to-maintain ground-based radio navigation aids. --Ken
