Projection Issues

[Neil Saunders]

Frank/Clint - Thanks for the pointers - Should keep me busy for a while!

On 2/9/06, Clint Johnson <cjohnson at refractions.net> wrote:
> 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.
> Cjohnson at refractions.net
>
>
> 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 <warmerdam at pobox.com> wrote:
> >
> >> On 2/7/06, Neil Saunders <n.j.saunders at gmail.com> 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, warmerdam at pobox.com
> >> light and sound - activate the windows | http://pobox.com/~warmerdam
> >> and watch the world go round - Rush    | Geospatial Programmer for Rent
> >>
> >>
>
>
>