QGIS – Creating Point on Surface in QGIS

centroidsogr2ogrpoint-creationqgisvector

As far as I can tell, the option to "Create point on surface for each part" in the QGIS Centroid tool is not returning the correct geometry when activated in QGIS (version 3.8.1-Zanzibar/GDAL v 2.2.3 on Debian).

If we take a simple "banana-shaped" polygon:

and run the tool (Vector > Geometry Tools > Centroids) as follows:

we get the following output, which represents the centroid, but not the point on surface.

We can cross check this output using ogr2ogr as follows:

ogr2ogr -dialect SQLite -sql "SELECT ST_Centroid(geometry) FROM tmp" tmp_centroid.shp tmp.shp

which returns the identical centroid:

whereas the following command forces the point on the surface of the polygon:

ogr2ogr -dialect SQLite -sql "SELECT ST_PointOnSurface(geometry) FROM tmp" tmp_ptonsurf.shp tmp.shp

is this a bug or is there something else that should be run/enabled in order to get the point on surface using the QGIS tool?

Best Answer

In QGIS 3.10 this option is not available using Vector > Geometry Tools > Centroids.

However if you search for "Point on surface" in the Processing Toolbox (Ctrl+Shift+T) using QGIS 3.10 the functionality is there and works.

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[GIS] sqlite version error with ogr2ogr

Since GDAL/OGR 1.10, the SQLite "dialect" can be used as an alternate SQL dialect to the OGR SQL dialect. This assumes that GDAL/OGR is built with support for SQLite (>= 3.6), and preferably with Spatialite support too to benefit from spatial functions.

Source: http://www.gdal.org/ogr/ogr_sql_sqlite.html

So you need to build GDAL (>= 1.10) against SQLite (>= 3.6) and SpatiaLite (>= 3.0) in order to use the -dialect SQLite option.

In order to understand which versions of SQLite and (eventually) SpatiaLite you are using, simply execute:

ogrinfo foo.shp -dialect sqlite -sql "select sqlite_version()"
ogrinfo foo.shp -dialect sqlite -sql "select spatialite_version()"

GDAL – How to Dissolve Naturalearth Vectordata with GDAL

This was a nice problem. Fortunately you gave excellent step-by-step instructions how to repeat the issue.

The last step with ST_Union does not fail and therefore you do not get any error message. The union is created but the function does not "dissolve" the parts but writes them as separate parts of a multipolygon. And that happens because the common line segment only looks like common. Closer look at the coordinates reveals that they are not equal. This is the left side:

180 64.97970870219838,  
180 68.96363636363661,

And this is the right side:

180.00000000000003 64.97970870219837, 
180.00000000000003 68.96363636363637,

Because of the small rounding errors ST_Union writes the left side and right side into separate members of a multipolygon. I do not know why it fails for both of us but not for @aflaciano. Perhaps we have different platforms or GDAL and Spatialite versions.

I made some further trials with good and bad news. Good news is that it is possible to combine the halves into one polygon with Spatialite by using a ST_SnapToGrid function. Bad news is that at least my GDAL dowloaded from gisinternals.com is not compiled with enough recent Spatialite and SnapToGrid is missing from it.

What should be done is to round the coordinates before executing ST_Union so that vertices match exactly. The command to use would be then

ogr2ogr world_output.shp world_shifted.shp -dialect sqlite -sql "select ST_Union(ST_SnapToGrid(Geometry,0.00001)),CNTRY_NAME FROM world_shifted GROUP BY CNTRY_NAME"

GDAL trunk version from gisinternals downloaded today (June 2nd, 2014) throws an error

no such function: ST_SnapToGrid

However, the command works with spatialite-gui v. 1.7.1 and the result is good.

Chukotka Peninsula though Spatialite and ST_Union + ST_SnapToGrid

Best Answer

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