RES: [postgis-users] User Friendly vs Explicit

[Gustavo Henrique Sberze Ribas]

  Some links about projections and the Earth:


 The Earth:
 http://www.gfz-potsdam.de/news/foto/images/GFZ-Potsdam-PR-20030122-Geoid-Undulation-1250x1250.jpg

 How to map the Earth:
 http://kartoweb.itc.nl/geometrics/Reference%20surfaces/body.htm 
 http://exchange.manifold.net/manifold/manuals/5_userman/mfd50The_Earth_as_an_Ellipsoid.htm
 http://solid_earth.ou.edu/notes/geoid/earths_geoid.htm

 Map Projections:
 http://www.fes.uwaterloo.ca/crs/geog165/mapproj.htm

 Check the Data Standards section:
 http://maic.jmu.edu/sic/topics.htm



--
Gustavo 


-----Mensagem original-----
De: postgis-users-bounces at postgis.refractions.net [mailto:postgis-users-bounces at postgis.refractions.net]Em nome de Mark Fenbers
Enviada em: sábado, 14 de maio de 2005 20:17
Para: PostGIS Users Discussion
Assunto: Re: [postgis-users] User Friendly vs Explicit


Mark Fenbers wrote: 
Norman Vine wrote: 
This is commonly referred to as the Vornoi Diagram
http://www.cs.cornell.edu/Info/People/chew/Delaunay.html

program that can compute this for you
http://www-2.cs.cmu.edu/~quake/triangle.html

  
Wow!  I never knew this had a name!  Your info was interesting and helpful, but I want to calculate Voronoi Diagrams using PostGIS so that all I have to pass around is some SQL to each site having different radars to work with...  I have yet to find documentation that makes it clear to me why SRIDs (other than -1) are useful, and how to reproject lat/lons into a planar projection.  I see that the distance() function also needs reprojected data to be useful...


OK, well, reading through more documentation and online stuff, I learned of a spatial_ref_sys.sql file that wasn't run during installation.  So now I have the spatial_ref_sys table populated and now understand that SRID relates to different map projections.  But this brings up even more questions, like "How in the world (figuratively) will I be able to pick an SRID that's right for the work I am doing?"  And is the use of SetSRID() what actually reprojects my data?

Mark