OpenStreetMap
I've mostly finished Aslockton and Whatton (Nottinghamshire, UK), and the changes have just appeared on the SlippyMap. Wooo!
I still need to join up a few footpaths, but then I'll move on to Orston, Scarrington, Elton...
Here's why mapping's so good:
* exercise
* fresh air
* reward myself with a sneaky pint of beer after mapping
* satisfy geeky tendancies
One curious thing: off Mill lane (northern part of Aslockton) there are some allotments and a cemetery. In JOSM, these are drawn with right-angles... but on the map they appear oblique. Odd...
Discussion
Comentari de Richard lo 8 d’octòbre 2008 a 21:52
Have you got the projection set to something weird in JOSM?
Comentari de flint lo 8 d’octòbre 2008 a 22:19
Thanks for the hint Richard :-)
"EPSG:4326". I assume this is the default, as I don't remember changing it. (But the "might have been drunk at the time" get-out-clause does apply)
I'm pretty sure it was an almost-exact rectangle on the ground, the GPX trace shows in JOSM as a rectangle... so that's how I drew the boundary. But it still looks wonky in the SlippyMap and in Potlatch :-/
Comentari de chillly lo 8 d’octòbre 2008 a 22:25
I think you need to set the projection to Mercator, which is the projection used on the slippy map.
Cheers, Chris
Comentari de flint lo 8 d’octòbre 2008 a 22:36
Aha - if I change the projection in JOSM to "Mercator", then the rectangles are oblique here too.
Ideally, I'd like to use a projection which closely represents what I'm seeing on the ground. So if I walk around a known regular shape, GPS in hand, that shape appears the same in JOSM.
Is there an easy choice?
Comentari de davidearl lo 8 d’octòbre 2008 a 23:36
EPSG:4326 is the default in JOSM for some peculiar reason, so you probably didn't change it, it was always like that.
Of course, if you change projection in JOSM the ones you've already done will now look distorted in the same way as they do on the slippy maps. If you walk around a building, do your GPS traces not appear with right-angles when loaded in JOSM? They should, because your GPS is just recording lat/lon and there is no projection involved at that stage.
Mercator preserves angles so a right angle on the ground should look right-angular in the Mercator projection, which is the one all our slippy maps are done in (though there's no reason they have to be, but it is computationally convenient and works well at large scales for exactly the reasons you;'re discovering).
At smaller scales, there is no right answer because it is all about representing a spherical surface on a flat plane and there are many ways of doing that.
Comentari de flint lo 8 d’octòbre 2008 a 23:44
Thank-you, everyone, for your comments. I've now changed to Mercator, and understand projections much better now.
I think I was just confused because the allotments and cemetery - which I now believe to be wonky on the ground- were erroneously appearing as regular rectangles in JOSM.
Comentari de peter taffs lo 21 d’octòbre 2008 a 13:39
hello from your East Bridgford neighbour. You've done a very nice job of Alsocton and Whatton. I notices a few other improvements, modifications and additions and they are very welcome.