Hi,
Near the bottom of this site is a book called "Understanding Map
Projections". It will explain just what projections are and why they
are used. Once you understand that you can proceed to semantically use
projections correctly.
http://fisher.lib.virginia.edu/reference/help/esri/esri_pdfs_9.html
Make sure you read chapter 1 which discusses Geographic Coordinate
Systems (GCS aka datum) as you will need to understand how they operate
in order to understand the purpose of a projection.
Clint
--
Clint Johnson
Refractions Research Inc.
[EMAIL PROTECTED]
Neil Saunders wrote:
Hi Frank - Thanks very much for your continued help. I'm still only a
the learning stage, and so am just having a play trying to get a
working system going.
For testing purposes, the shapefile file I'm using is here:
http://www.cdc.gov/epiinfo/shape/Cntry00.zip
What I'd like to do is load this shape file in to a PostGIS database,
then have mapserver render it projected as mercator (WGS84?),
overlayed with points I've got in another table (specified as
Lat/Long). As you can probably tell, most of my lack of understanding
revolves around projections - Are there any good resources you could
point me towards regarding this?
Kind Regards,
Neil.
On 2/7/06, Frank Warmerdam <[EMAIL PROTECTED]> wrote:
On 2/7/06, Neil Saunders <[EMAIL PROTECTED]> wrote:
Hi All!
Frank; Thanks for that tip, now I'm setting the projection on the layer and
mapserver is doing my translation into merc which is great. However, the
units its projecting it into are meters. If I set, as I have, the units
in the layer projection to be m and the units of the map to be dd then i
get a "[MapServer Error]: msProcessProjection(): unknown unit conversion
id" error.
Neil,
If your map is going to be in mercator you are going to
have to specify your extents in some linear measure (like
meters), not in lat/long.
So I tried to convert my data into a new shapefile so i could re-import
it, using the following command:
ogr2ogr -s_srs "+proj=latlong" -t_srs "+proj
=merc +ellps=WGS84" e:\upload\worldmerc.shp e:\upload\cntry.shp
ERROR 1: Failed to find required field in datum.csv in
InitDatumMappingTable(),
using default table setup.
Failed to transform feature 180.
ERROR 1: Terminating translation prematurely after failed
translation of layer cntry00
I'm not exactly sure why you get this error. But you really
should be specific about datum.
eg.
ogr2ogr -s_srs "+proj=latlong +datum=WGS84" -t_srs "+proj=merc
+datum=WGS84" e:\upload\worldmerc.shp e:\upload\cntry.shp
I would add it really frightens me when you leave
so many mercator parameters defaulted.
Best regards,
--
---------------------------------------+--------------------------------------
I set the clouds in motion - turn up | Frank Warmerdam, [EMAIL PROTECTED]
light and sound - activate the windows | http://pobox.com/~warmerdam
and watch the world go round - Rush | Geospatial Programmer for Rent
