GIS / PostGIS ?

[Tom]

Is there anyone here who has experience with GIS, or even specifically PostGIS? I know there are a few people with GIS knowledge in the BM community, but I didn't keep a list somewhere. :-(

[Chenier]

Is there anyone here who has experience with GIS, or even specifically PostGIS? I know there are a few people with GIS knowledge in the BM community, but I didn't keep a list somewhere. :-(

As I've stated elsewhere, I have experienced with GIS. Not PostGIS per say, though, but the website says:

What is PostGIS?
PostGIS adds support for geographic objects to the PostgreSQL object-relational database. In effect, PostGIS "spatially enables" the PostgreSQL server, allowing it to be used as a backend spatial database for geographic information systems (GIS), much like ESRI's SDE or Oracle's Spatial extension. PostGIS follows the OpenGIS "Simple Features Specification for SQL" and has been certified as compliant with the "Types and Functions" profile.

I used ESRI's SDE, which was liked to an Oracle database... Limited experience with other GIS software, though.

And as stated in the other threat, variable availability and motivation.

[Solari]

More or less my entire academic and professional career is built upon or has involved GIS.

[Tom]

Ok, here is a question for the GIS experts:

I use a PostGIS database and OpenLayers.

Is there a way to teach these two that I am using flat geometry, not a sphere/earth? Alternatively, what's the best way to merge vector data I have in PostGIS with an image I generated from that data and rastered with gdal2tiles.py ?

Imagine that the region polygons for BM get used to generate a map in PNG format, and then overlayed over that same result. For some reason, there seems to be a very odd relation between pixel and GIS coordinates I don't understand. Seems to be something with a degree/metres relation or whatever. So how do I translate between these?

[Chenier]

It sees your pixels, and assumes they are unknown "units". Depending on the software and your settings, it may keep them in "unknown units", or assume they are units of a certain default projection. As such, it views the pixel grid of your raster as some kind of geographic positioning grid, from my understanding.

I can imagine that such behavior, if you have large pixels (and therefore a large "unknown unit" grid), could create distortions in the extremities. My intuition is that this is more likely to happen with a system that uses degrees, minutes, seconds, etc, than a metric cartesian projection like UTM (my apologies, I learnt this in French and even in French I have a hard time remembering the technical descriptions).

Then again, when importing your maps into both ArcGIS (proprietary) and QuantumGIS (open source), I never had such issues, and I could draw forms on top of it without any kind of problem. Perhaps the geodatabase forces you to select a projection, though, which could be a source of problems. I'll have to look more into it. I apologize for not having taken the time to do so yet, I figure I should have some time soon to do so.

[Tom]

I've since figured it out mostly by trial and error and some reading. I now have proper scaling, yeah!

Do you know anything about GeoServer
http://geoserver.org/display/GEOS/Welcome
?

I'm not sure if I should use that or my own code to serve my vector data. It's a bit heavy with Tomcat and all that, but probably a lot more powerful than anything I could hack up.