[Openstreetmap] experiments with the New Popular Edition

[Jo Walsh]

this morning i hauled out the New Popular Edition CD that Richard
kindly sent us for the Map Room, and spent a couple of hours fiddling
around trying different ways of georeferencing it. i wanted to share
how far i'd got here and outline plans for it...

http://uo.space.frot.org/freemap/popular.cgi is the north-east london
sheet, slung into mapserver over a scrap of Landsat; the yellow
squiggle at bottom left shows us walking down Limehouse Cut, so you
can see it is a little bit offset against the blue line below.

plans:

- i'd like to do the four London sheets and offer a NPE base layer as
  part of a london free map service
- Schuyler has a half-finished GUI client for georeferencing; i might
  wait for that rather than doing the other 3 by hand. 
- we will have a 'lab' soon in the Map Room and would like to do some
  free map workshops over late may and june; this could be a session...

process:

i went through a manual process of creating GCPs by reading off the
grid lines and moving the mouse pointer over them, in the GIMP, 
to get the x and y pixel values, which is what produced the slight
error, my sloppiness. Trying a linear transform with the 
four corners didn't work out, so i had to use ten GCPs, which kicks
gdal_translate into quadratic transform mode. 

jo at frot:~$ gdal_translate -a_srs "+init=epsg:27700" -gcp 95 156 535000
225000 -gcp 2580 133 575000 225000 -gcp 2607 2919 575000 180000 -gcp
112 2939 535000 180000 -gcp 409 526 540000 219000 -gcp 545 2501 542000
187000 -gcp 1042 2436 550000 188000 -gcp 1854 2371 563000 189000 -gcp
1965 638 565000 217000 -gcp 1468 825 557000 214000 london2.tif
london2_gcp.tif
jo at frot:~$ gdalwarp london2_gcp.tif london2f.tif

i'd be thrilled to receive feedback about this activity before i do
too much more of this...


-jo