I am quite new to GIS and OSM and am trying to get my head around exporting custom OSM region and uploading them into PostgreSQL.
I have started with a very small map area to export with only two shapes – an island and a meadow – (http://www.openstreetmap.org/#map=19/43.86436/15.34266) which export the following .osm data:
<?xml version="1.0" encoding="UTF-8"?>
<osm version="0.6" generator="CGImap 0.3.3 (23630 thorn-02.openstreetmap.org)" copyright="OpenStreetMap and contributors" attribution="http://www.openstreetmap.org/copyright" license="http://opendatacommons.org/licenses/odbl/1-0/">
<bounds minlat="43.8635800" minlon="15.3411900" maxlat="43.8650000" maxlon="15.3445500"/>
<node id="63910084" visible="true" version="3" changeset="17709161" timestamp="2013-09-06T20:07:53Z" user="idelac" uid="183676" lat="43.8644519" lon="15.3432012"/>
<node id="63930603" visible="true" version="3" changeset="17709161" timestamp="2013-09-06T20:07:54Z" user="idelac" uid="183676" lat="43.8642369" lon="15.3431567"/>
<node id="63968158" visible="true" version="3" changeset="17709161" timestamp="2013-09-06T20:07:57Z" user="idelac" uid="183676" lat="43.8645365" lon="15.3424617"/>
<node id="1122490202" visible="true" version="2" changeset="17709161" timestamp="2013-09-06T20:07:44Z" user="idelac" uid="183676" lat="43.8640377" lon="15.3426171"/>
<node id="63879158" visible="true" version="3" changeset="17709161" timestamp="2013-09-06T20:07:52Z" user="idelac" uid="183676" lat="43.8639894" lon="15.3430113"/>
<node id="2448417703" visible="true" version="1" changeset="17709161" timestamp="2013-09-06T20:06:52Z" user="idelac" uid="183676" lat="43.8641440" lon="15.3427266"/>
<node id="2448417704" visible="true" version="1" changeset="17709161" timestamp="2013-09-06T20:06:52Z" user="idelac" uid="183676" lat="43.8641542" lon="15.3426165"/>
<node id="2448417706" visible="true" version="1" changeset="17709161" timestamp="2013-09-06T20:06:52Z" user="idelac" uid="183676" lat="43.8642112" lon="15.3428451"/>
<node id="2448417707" visible="true" version="1" changeset="17709161" timestamp="2013-09-06T20:06:52Z" user="idelac" uid="183676" lat="43.8642458" lon="15.3424979"/>
<node id="2448417708" visible="true" version="1" changeset="17709161" timestamp="2013-09-06T20:06:52Z" user="idelac" uid="183676" lat="43.8643394" lon="15.3428931"/>
<node id="2448417709" visible="true" version="1" changeset="17709161" timestamp="2013-09-06T20:06:52Z" user="idelac" uid="183676" lat="43.8644045" lon="15.3425008"/>
<node id="2448417711" visible="true" version="1" changeset="17709161" timestamp="2013-09-06T20:06:52Z" user="idelac" uid="183676" lat="43.8645022" lon="15.3428874"/>
<node id="2448417712" visible="true" version="1" changeset="17709161" timestamp="2013-09-06T20:06:52Z" user="idelac" uid="183676" lat="43.8645144" lon="15.3425403"/>
<node id="2448417713" visible="true" version="1" changeset="17709161" timestamp="2013-09-06T20:06:52Z" user="idelac" uid="183676" lat="43.8645937" lon="15.3427745"/>
<node id="2448417714" visible="true" version="1" changeset="17709161" timestamp="2013-09-06T20:06:52Z" user="idelac" uid="183676" lat="43.8645978" lon="15.3426673"/>
<node id="1122490153" visible="true" version="2" changeset="17709161" timestamp="2013-09-06T20:07:43Z" user="idelac" uid="183676" lat="43.8645802" lon="15.3431461"/>
<node id="1122490171" visible="true" version="2" changeset="17709161" timestamp="2013-09-06T20:07:43Z" user="idelac" uid="183676" lat="43.8646997" lon="15.3426923"/>
<node id="1122490322" visible="true" version="2" changeset="17709161" timestamp="2013-09-06T20:07:44Z" user="idelac" uid="183676" lat="43.8647101" lon="15.3429354"/>
<node id="1122490460" visible="true" version="2" changeset="17709161" timestamp="2013-09-06T20:07:45Z" user="idelac" uid="183676" lat="43.8641347" lon="15.3424630"/>
<node id="1122490524" visible="true" version="2" changeset="17709161" timestamp="2013-09-06T20:07:45Z" user="idelac" uid="183676" lat="43.8642836" lon="15.3424149"/>
<way id="8837893" visible="true" version="3" changeset="7098380" timestamp="2011-01-26T21:37:50Z" user="goranT" uid="217969">
<nd ref="63879158"/>
<nd ref="63930603"/>
<nd ref="63910084"/>
<nd ref="1122490153"/>
<nd ref="1122490322"/>
<nd ref="1122490171"/>
<nd ref="63968158"/>
<nd ref="1122490524"/>
<nd ref="1122490460"/>
<nd ref="1122490202"/>
<nd ref="63879158"/>
<tag k="natural" v="coastline"/>
<tag k="place" v="island"/>
<tag k="source" v="PGS"/>
</way>
<way id="236881851" visible="true" version="1" changeset="17709161" timestamp="2013-09-06T20:07:26Z" user="idelac" uid="183676">
<nd ref="2448417709"/>
<nd ref="2448417707"/>
<nd ref="2448417704"/>
<nd ref="2448417703"/>
<nd ref="2448417706"/>
<nd ref="2448417708"/>
<nd ref="2448417711"/>
<nd ref="2448417713"/>
<nd ref="2448417714"/>
<nd ref="2448417712"/>
<nd ref="2448417709"/>
<tag k="landuse" v="meadow"/>
</way>
</osm>
Running osm2pgsql -S default.style -d gis -U postgres island.osm uploads the data into my gis database with one row in the planet_osm_line table (for the island) and one row in the planet_osm_polygon table (for the meadow).
Now, when issuing sql query SELECT ST_AsText(way) FROM planet_osm_line, pqsql returns
"LINESTRING(1707992.39 5444456.51,1707997.34 5444489.7,1707915.02 5444502.76,1707932.32 5444425.75)"
Here I don’t understand two things-
Questions
- Why does the Linestring only contain four points when the original coastline of the island has 10 distinct nodes? Similarly when issuing the sql query on the planet_osm_polygon table to see the polygon for the meadow shape, the polygon has fewer coordinates than its original shape described by the nodes (7 instead of 11).
- (answered just below in update) How are the original lat,lng coordinates projected? I know osm2pgsql defaults to mercator projection. However, I don’t understand how it computed the values in the Linestring above. Is the width of a tile (256px?) and some zoom (z=?) taken into account in the projection? Would LatLng[0,0] be projected as a Point[0,0]?
Update
The answer to my second question is that osm2pgsql projects from latitude and longitude coordinates to mercator in meters. LatLng[0,0] will indeed be projected as Point[0,0] where the point coordinates are the distance in meters from the origin on a mercator plane.
Here is the projection function in case someone stumbles upon this question and looks for a solution
var degrees2meters = function(lon,lat) {
var x = lon * 20037508.34 / 180;
var y = Math.log(Math.tan((90 + lat) * Math.PI / 360)) / (Math.PI / 180);
y = y * 20037508.34 / 180;
return [x, y]
}
I still don’t know why some of the nodes are missing in the resulting shapes in the database. Has anyone experienced this?