[Mapserver-users] Map projections

[Krzysztof Chodak]

Hello again dear mapserver professionals,

I would be grateful if someone could clear my mind in projections subject. (I'm sorry for such inquiries but it is too difficult for me to solve these problems and maybe they would be simple for some of you)


I got all geographic data in WGS84 projection. As I have made some "investigation" I think that Mercator is proper projection for my target users and theirs needs (I need to show all Europe road network for routing purposes). Am I correct that it is most suitable for such area and planned usage?

Is there any significant difference in ellipsoid used with Mercator projection? In EPSG guide Bessel is used in an example. Is it suggested ellps for Mercator (SP1) or may I simple stay with WGS84 one? Should I recompute false easting and northing given in example when switching to WGS84 ellps? Maybe they are constants, aren't they?

I also cannot catch the difference between one and two standard parallels. Which one is more suitable for my case in your opinion?

Another thing is a k0 parameter (scale factor at natural origin). When I'm doing following transformation (using PROJ library):
from "+proj=latlong +datum=WGS84"
to "+proj=merc +ellps=bessel +lat_ts=0N +lon_0=110E +k_0=0.997 +x_0=3900000 +y_0=900000"
	or the same with ellps=WGS84
the k0 parameter is replaced by 1.0 (and practically skipped in next computations as it is used as multiplier) after initiating target projection (pj_init). Is it correct way of PROJ transformation and would it have any influence on my projected maps when k0 factor would be skipped and not set to 0.997?

Maybe you can also give some advices for lat_ts and lon_0 - should I change them to any other values to project my area more precisely?


I got also a polite question to Jan Hartmann [jhart at frw.uva.nl] if he could publish his world map projections images in our discussion group - maybe they would help me and other people with understanding projections stuff?

Best regards,

Krzysztof Chodak

...lost in projections world :)


PS. Example from EPSG guide:
"2. Mercator (1SP) (EPSG coordinate operation method code 9804)
For Projected Coordinate Reference System: Makassar / NEIEZ
Parameters:
Ellipsoid: Bessel 1841 a = 6377397.155metres 1/f = 299.15281
then e = 0.081696831
Latitude of natural origin ϕO     0°00'00"N = 0.0 rad
Longitude of natural origin λO 110°00'00"E = 1.91986218 rad
Scale factor at natural origin kO 0.997
False easting FE 3900000.00 metres
False northing FN   900000.00 metres
Forward calculation for:
Latitude ϕ =     3°00'00.00"S = -0.05235988 rad
Longitude λ = 120°00'00.00"E = 2.09439510 rad
gives Easting E = 5009726.58 metres
Northing N =   569150.82 metres
Reverse calculation for same easting and northing first gives:
t = 1.0534121
χ = -0.0520110
Then Latitude ϕ =    3°00'00.000"S
Longitude λ = 120°00'00.000"E"