--- On Thu, 25/6/09, James Livingston <doc...@mac.com> wrote: > Because I'm a bit bored, I thought about the above. Assume > you want an > accuracy of 1m and 1s:
Most use 6dp (about 10cm) because it fits neatly in an int, but yea 1m or even multiples of 2 or 3m might be good enough since most GPS won't do better than 2 or 3m accuracy. > stored in it's fully glory, using say the 13 byte encoding > mentioned If you using delta's you only need 12 bytes, since you in theory wouldn't go over 65,535 seconds. Going down this path you would put the full time stamp in the header. > * the DOP has changed Why would this matter, DOP usually varies, although it is usually pointless recording over 4 or 5, and if you want to 1dp multiple it by ten, so you only really need +/-60 so plenty of room to move in a single byte. > If the first byte is not -128, then it is the change in > altitude since > the last point. The second byte is the change in latitude > the third > byte is the change in longitude, and the fourth is the > number of > seconds since the last point. Most altitude won't change by +/-60m that quick unless you're on a Qantas flight :) Seriously though, vdop isn't any where near as acurate as hdop, so you could do multiples of 10m. > There you go, most points now take up 4 bytes :) in which case you can start doing nibbles instead of bytes, and you might be able to get it down to 2 bytes if you really try. > But I'm not that bored. Me either, I had to mess about with nibbles and even single bits in the past and it's not fun. While I agree with your sentiments about GPX, I think some variation of JSON would be more efficient from a programming point of view and potentially just as human readable. I made a post to this effect on the GeoJSON list, they have a basic spec nutted out, but it's lacking when it comes to most of the valuable information stored in GPX files like time stamps, elevation and hdop. If you are bored, the full details of my post can be found here: http://lists.geojson.org/pipermail/geojson-geojson.org/2009-June/000488.html _______________________________________________ Talk-au mailing list Talk-au@openstreetmap.org http://lists.openstreetmap.org/listinfo/talk-au
