[Mapserver-users] Map projections

[Krzysztof Chodak]

Thanks for Your helpful information.

Yes, You are right - I got raw non-projected long/lat data.
As I have understood You suggest staying with WGS84 datum to minimize errors.

Do You happen to know how should Mercator parameters look like? 

Best regards,

Krzysztof Chodak

-----Original Message-----
From: mapserver-users-admin at lists.gis.umn.edu [mailto:mapserver-users-admin at lists.gis.umn.edu] On Behalf Of Gerry Creager N5JXS
Sent: Thursday, March 27, 2003 12:45 AM
To: Krzysztof Chodak; mapserver-users at lists.gis.umn.edu
Subject: Re: [Mapserver-users] Map projections

I'm still catching up on today's e-mail, so someone else may answer or 
have answered you, better than I.  But, here goes...

A projection is a mathematical tool used to represent an image of data 
normally meant to be shown on a spherical (or ellipsoidal) 
representation, in a flat display.  If you were to take your world 
globe, cut the map covering it, and remove the image from the globe, 
when you lay it out on the table, it's no longer spherical and not a 
rectangle.

Most of our images are rectangular in general shape, so we try to 
display our maps to fit that structure.  In order to do this, we have 
developed a number of "projections" designed to demonstrate the data 
with less distortion than we'd see if it remained unprojected.

In your note, you state that you are using a WGS84 projection.  However, 
at best, with the information you provide, I suspect you're using a 
non-projected longitude/latitude "projection" which does not diminsh 
distortion, but rather, will provide discrepancies all through your image.

In fact, WGS84 is a datum, the reference frame for geodetic measurement. 
  WGS84 is a dynamic datum designed to accommodate GPS use throughout 
the world.  Its ellipsoid is consistent with the ellipsoid referenced 
for NAD83, although not identical.  It is not the same as a Bessel 
ellipsoid.  If your data were acquired with WGS84, and your metadata so 
state, you need to reproject it in WGS84. The datum will affect accuracy 
of the representation; the projection will reflect the distortion in the 
display.  If your data were acquired using WGS84, and you tell proj 
'bessel' then things will likely be a bit erroneous.

Mercator projection should be satisfactory for displaying Europe, 
although there may be a bit of distortion at map edges.

gerry

Krzysztof Chodak wrote:
> 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"
> 
> 
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-- 
Gerry Creager -- gerry.creager at tamu.edu
Network Engineering -- AATLT, Texas A&M University	
Cell: 979.229.5301 Office: 979.458.4020 FAX: 979.847.8578
Page: 979.228.0173
Office: 903A Eller Bldg, TAMU, College Station, TX 77843

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