On 8/9/2013 12:03 PM, Joseph Marlin wrote:
The Case of the Disappearing Roads
We create shapefiles with SQL querying world data that's been loaded into a
PostgreSQL database. A python script requests tiles from mapserver at different
zoom levels, and we generate the entire map like that.
Now, at the sixth zoom level, level 3 roads (medium size roads) should be
rendered. However, they are actually rendered only in a small section of the
world, a box bounded by Toronto in the northwest, Cleveland in the southwest,
and the Atlantic in the east. Elsewhere, level 3 roads are not rendered at all.
As you can see in this image: http://i.imgur.com/6McvGOJ.png, the small gray
roads, level 3 roads, (and highway shields, for that matter) that are visible
on the top half suddenly stop being rendered by mapserver.
In all the following more zoomed in levels, no roadways smaller than level 2
are rendered at all anywhere, as you can see here:
http://i.imgur.com/8MTcPoi.png. I've verified that the shapefiles contain the
data on the smaller roads as I have viewed them just fine with QGIS, as you can
see here: http://i.imgur.com/S3W3Iy8.png
I'm so confused as to what could possibly make roads disappear midway through a
level. If it was a problem with my mapfile, why would they show up in part of
Northeast US, but not anywhere else? Where do I even start looking to solve
this?
Thanks so much!
Additional info:
SRS: EPSG:900913
Zoom levels: 19567.8792375, 9783.93961875, 4891.969809375, 2445.9849046875,
1222.99245234375, 611.496226171875, 305.7481130859375, 152.87405654296876,
76.43702827148438, 38.21851413574219, 19.109257067871095, 9.554628533935547,
4.777314266967774, 2.388657133483887, 1.1943285667419434, 0.5971642833709717,
0.29858214168548586
Bounding box: -20037508.3427892,-20037508.3427892,
20037508.3427892,20037508.3427892
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I've seen that problem when the data is invalid. Try something like:
select count(*) from roads where not ST_IsValid(the_geom);
You should get zero.
-Andy
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