Tech:Geography

From Cyclopath

• Geographic data is stored, transferred, and displayed using NAD83-based UTM Zone 15N coordinates (EPSG spatial reference system ID 26915).
• Map scale is expressed in screen pixels per meter.
• Zoom level z maps to scale s as s = 2^{z − 16}, e.g. zoom level 1 has scale 1 pixel/32.768 kilometers, zoom level 10 has scale 1 pixel/64 meters, and zoom level 15 has scale 1 pixel/2 meters.
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  Screen and map coordinates have opposite y coordinate polarity.
  Note that screen coordinates and map coordinates have the y coordinate increasing in opposite directions: increasing downwards on the screen and upwards on the map. See at right.
• Geometry is moved between the database to the client in a bare-bones text format; the geometric type is defined by object type. Currently this is an SVG-based hack.
• Geometry is moved from the client to the database in WKT format, though some portions are implied during transport and added by the server.