Mapping Hacks

Load Your Waypoints into a Spatial Database #88

Chapter 8, Building the Geospatial Web

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HACK

OGC SRS is utterly arcane. You want 4326 because it means

EPSG 4326, which means geographic coordinates referenced

to the WGS 1984 ellipsoid and datum. We know that 4326

is the WGS 1984 ellipsoid because it is defined in the

spatial_ref_sys table. Run select * from spatial_ref_sys

where srid = 4326;

inside the psql shell to see how PostGIS

defines this SRID. Take note of the column devoted to

PROJ.4 parameters: this is how PostGIS knows what to do

with this particular spatial reference ID. You need to under-

stand the spatial reference options if you want to master

PostGIS, but for now, sit back, relax, and trust the hack.

We can combine the conversion of waypoints with loading the database

using the Unix tool chain:

./parse_gpx_way.pl gpx_waypoints_file.gpx | psql -d gpswork

Congratulations! You now have waypoints loaded into a spatial database.

Let’s start PostGIS with the command

psql gpswork and take a look at what

we can do:

gpswork=# select name, cmt, descr from waypoint order by name;

name | cmt | descr

--------+----------------------+----------------------

100 | 100 MILES | 100 MILES

588 | CRTD 14:58 06-SEP-04 | CRTD 14:58 06-SEP-04

ABSPOT | CRTD 10:15 08-SEP-01 | CRTD 10:15 08-SEP-01

AC1 | CRTD 15:04 16-AUG-00 | CRTD 15:04 16-AUG-00

...

Calculating Distances in PostGIS

Let’s say we want to calculate ...