OpenStreetMap 開放街圖
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...
討論
由 Richard 於 2008年10月 8日 21時52分 發表的評論
Have you got the projection set to something weird in JOSM?
由 flint 於 2008年10月 8日 22時19分 發表的評論
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 :-/
由 chillly 於 2008年10月 8日 22時25分 發表的評論
I think you need to set the projection to Mercator, which is the projection used on the slippy map.
Cheers, Chris
由 flint 於 2008年10月 8日 22時36分 發表的評論
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?
由 davidearl 於 2008年10月 8日 23時36分 發表的評論
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.
由 flint 於 2008年10月 8日 23時44分 發表的評論
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.
由 peter taffs 於 2008年10月21日 13時39分 發表的評論
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.