Projection Issues

[Clint Johnson]

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
>>
>>