[Proj] Why?

Frank Warmerdam warmerdam at pobox.com
Mon Oct 27 11:30:16 EDT 2008

Gerald I. Evenden wrote:
> On Monday 27 October 2008 9:48:36 am support.mn at elisanet.fi wrote:
>> "Gerald I. Evenden" <geraldi.evenden at gmail.com> kirjoitti:
>>> One burning question has come up in my mind about the need for inverse
>>> solutions for non-grid-system projections like Aitoff, Hammer, etc.: why?
>> For our part we need the inverse for every projection, since the users want
>> to get their coordinates and other information just by clicking on the
>> maps. For us those projections without forward and inveres are not usable
>> at all. We currently simply discard all projections which do not have
>> inverses. So we need them both all the time. And of course in the view
>> point of having a general projection package there could be all the
>> inverses also defined as far as it is practical.
>> Regards: Janne.
> I cannot say that I understand your practices but outside of cadastral 
> operations I do not see the usefulness of more than a four or five 
> projections---even less for screen display.
> Really, for point and click operations one only needs equidistant cylindrical 
> and polar.
> For publication use anything that suits the purpose because you do not need to 
> retrieve data from such output.
> As I said before, I do not comprehend the why of the "needs" of your users 
> and/or the item you are selling.


I'm with Janne on this.  In typical GIS applications (I am often dealing
with users of UMN MapServer) it is necessary for the projections to be
invertable in order to use them systematically.   From a practical point of
view there is little point in asking why folks use a particular projection
in a particular way.  If the projection exists it will get used in a
variety of ways - some of which are objectively somewhat silly.

But to be well behaved for applications built on it, it is very helpful
for the library to support inverses for all projections that might get

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

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