2008/04/20

GIS For Web Developers

Scott Davis on Seattle NFJS Java Conference


ACT1 - Free Data Available
- Raster Data, Google Satellite
- Vector Data, Google Map
- Hybrid, Google Satellite + Google Map Hybrid mode

Type of Vectors
- Point
- Line
- Polygon

Free Vectors: www.census.gov

Shapefile: well documented proprietary ESRI standard, .shp, .shx, .dbf

 

More data, please: http://www.nationalatlas.gov


For local data, most state and municipal government agencies offer free data as well. 

ACT2 - Projections

Earth is round but maps are flat, projections take fundamentally 3d data and portraying it in 2-dimensions, this procedure introduce error for distance, direction, shape, area. On a Cartesian Planes, X and Y lines form perfect squares, however, on a globe, only the line of Latitude are perfectly parallel, line of Longitude converge at the poles. Different projections attempt to minimize map distortions, common projections are "State Plane" and "UTM".

Reprojection Utility available: GDAL 


ACT3 - Spartial Databases

Why bothers with a database?
- Centralize many scattered files
- Provide security
- Indexing
- Cross dataset queries

PostgreSQL + PostGIS: http://postgis.refractions.net
Visualizing the Data: uDig

ACT4 - Web Services

- Google Maps, undeniably cool, proprietary interface
- Open set of standards available: Open Geospatial Consortium (OGC)
- OGC Features: GetCapabilities, DescribeFeatureType, GetFeature
- Data Service: WFS (Web Feature Service), WMS (Web Mapping Service)
- Open standard, any one can implement them: http://opengeospatial.org/resource/products
- GeoServer: Java based, runs in a Servlet container, provides OGC web services
- MapBuilder: Pure Javascript web client for OGC, included with GeoServer
- OpenLayers: Pure javascript web client, supports OGC, Google Maps, Yahoo Maps, MS Virtual Earth and WorldWind

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