[aprssig] APRS LAT/LONG standards

VE7GDH ve7gdh at rac.ca
Sat May 28 05:55:01 EDT 2005

Andre PE1RDW schreef:

> you mean 400 grad format? I have been using that for decades in my
> profecional carreer along with co-ordinates in meters from a point just
> above paris, known as the dutch "rijks driehoeks net".

I'm not familiar with "rijks driehoeks net" but it sounds like it has a lot
going for it once you know how it works.

> Talking about gps standards, what datum are we using because in europe
> wgs is default but I think the american continent has a different datum
> as has  asia.

In BC Canada, the topo maps that I have worked with up until now have been
NAD27... North American Datum based on 1927 surveys. I understand that new
maps (at least here) will be WGS84 or possibly NAD83 instead when they are 
printed. Around here, there is something like 100 metres difference 
east-west and about 200 metres north-west. When I was with search and 
rescue, one of the things we had to deal with was that the helicopters had 
their GPS receivers set to WGS84. This is going back about 6 years, so 
between that and selective ability, there was the potential for some fair 
sized errors. As I mentioned in my earlier message, it was easy to change 
the format in the GPS if needed, and it was also easy to change the datum in 
the GPS if I needed to translate something.

You are probably familiar with the UTM that I mentioned. Universal
Transverse Mercator is based on 60 zones that are 6º (latitude) wide and
with the "easting" and "northing" in metres for the coordinates. It is a
very easy one to use with paper maps that have the UTM grid printed on them.
At the speed that helicopters and other aircraft can fly at, and the
distances they can cover, it makes more sense to use lat / long, but there
was a recent message from someone that mentioned that the helicopters they
work with for SAR are now using UTM.

For APRS, the "standard" is WGS84. In spite of that, if you are referencing
maps calibrated with another datum, I would think you would be better off
using that same datum in the GPS. I wasn't sure if a GPS always puts out
WGS84 in NMEA mode or if it puts out a position based on the datum that the
GPS is set to. I just did a search and came across a page that said "older
GPS Receiver firmwares always output WGS84, independent of the chosen map
datum..." so I assume that means that newer receivers might put out
coordinates based on whatever datum the receiver is set for. If this is the
case, setting your GPS for APRS to your local datum would mean that there
would be errors in your position on the APRS-IS & findu.com etc. if that is
the case. When it comes down to it, the errors will be small if you are
looking at a map the size of a city. However, if you were zoomed down to
look at an area of just a few city blocks, it could be significant.

73 es cul - Keith VE7GDH
"I may be lost, but I know exactly where I am!"

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