Notes on providing 1km tiles and a WMS for Out of Copyright Nothern Ireland maps.
Measuring the lat/lng coordinates of the corners of the maps gave values that were internally inconsistent assuming the maps were produced using bonne projection. Actual positions were reasonable, but in some cases the corners could be inconsistent by up to 150m.
Maps 5 and 8 which are externally gridded with a 1km grid have lat/lng values measured from the maps that agree to typically 10m or less with the measured grid references for the corners.
Decision taken to work from top left map 8 to deduce the theoretical locations of the other maps. Map offsets are calculated from the measured lat/lng for the top left of the maps, turning these into offsets in miles, rounding to integer miles, or occasionally sensible fractions (1/4 1/2 3/4 1/3 2/3)
Projection parameters used:
old maps assumed to be projected using
+proj=bonne +ellps=airy +x_0=200000 +y_0=250000 +lat_1=53.5 +lon_0=-8 +no_defs
Tiff world files were produced for the maps and loaded into mapserver. TFW files were used because they can easily be changed unlike gtiff which requires that large tiff files be rewritten on every adjustement. Comparing these WMS maps to landsat gives errors in the region of 100m.
Further looking on the web suggests a helmert 7 parameter transformation for current OSNI is
482.530, -130.596, 564.557, -1.042, -0.214, -0.631, 8.15
Previous experience with OSGB 6th edition maps has shown that offsets are much improved if +towgs84 parameters are used.
Use of these 7 parameters has not yet been implemented.
Plan 14/May/2007:
- get ponies WMS to serve NI maps (with overlays)
- ../../FWTools-1.2.1/bin_safe/gdaltindex ni_allt.shp /home/snaffle/gis/maps/NIBonne/*.tiff
- adjust ponies to use +towgs84 parameters
- compare against landsat and possibly adjust positions to get a better correspondence.
ponies niallt WMS now working with +towgs84 parameters
Looking at Belfast area using JOSM:
vertical registration looks good, horizontal is out by approx 300m too far left. JOSM agrees with Landsat
Looking at Ross Point in Lough Melvin (near TL map 
error approx 60m mostly horizontal
Adjusted belfast 200m to right – looks a lot better.
Reset back to original values. TL Map 7 differs with landsat by 200m is as well.
Comparing against google maps….. (naughty, naughty)
Main street junction in Garisson (near TL map
Google:
-8.090121746063232, 54.415215441843706 (OSNI:194149.64 351787.17)
Map (mod_airy, towgs):
-8.091774725654096, 54.414449515840396 (OSNI:194042.22 351702.06)
Map (mod_airy, nowgs)
-8.090979120218032, 54.41501103830758 (OSNI: 194093.95 351764.49)
map is 107m to the West, and 85m South as compared against google with wgs conversion
map is 55m to the West, and 22m South as compared against google without wgs conversion
—-
Bottom left of Map 8
Road junction near Derrynahinch (near corry island)
Google -8.101719617843628 54.14122807273472 (OSNI:193352.80 321293.12)
Map (wgs) -8.102291770360754 54.141412617001365 (OSNI:193315.44 321313.71)
Map (no wgs) -8.101477141739759 54.14197498375662 (OSNI:193368.76 321376.23)
map is 37m to the West, and 20m North as compared against google with wgs conversion
map is 15m to the West, and 83m North as compared against google without wgs conversion
—–
map 7
Google: -8.114004135131836 54.7220921576456 (OSNI:192654.67 385946.76)
towgs: -8.112944511768879 54.71969098394593 (OSNI:192722.51 385679.37)
no towgs: -8.112197731815117 54.72029205073798 (OSNI:192770.73 385746.20)
towgs: 67 East 267 South
no towgs: 116 East 200 South