GeoDjango: strip z dimension; force 2D

[Jônatas Castro]

In my project I need to import shapefiles and same some geometry.

Some of these are MULTIPOLYGON Z type, but all Z coordinates are 0-value.

When I try to save the geometry, I get the error: "Geometry has Z dimension but column does not".

What is the best way to strip the Z dimension?

My code:

ds = DataSource(file_path, encoding='ISO-8859-1')
layers = ds[0]

#need something HERE to coerce geometry to 2D

obj=MyModel(geometry=GEOSGeometry(layers[0].geom.hex))

obj.save()

[Alex Mandel]

1. There a GeoDjango sublist, better place to ask.
2. Strip out the Z when doing the database import, you don't say which
db backend your're using.
2a. If you can't do it on import than do it with an update query.

http://gis.stackexchange.com/questions/78142/how-can-i-strip-out-z-values-in-postgis

Thanks,
Alex

On 03/07/2016 10:36 AM, Jônatas Castro wrote:
> In my project I need to import *shapefiles* and same some geometry.
>
> Some of these are *MULTIPOLYGON Z* type, but all Z coordinates are 0-value.
>
> When I try to save the geometry, I get the error: *"Geometry has Z
> dimension but column does not".*
>
> *What is the best way to strip the Z dimension?*
>
>
>
> My code:
>
>
>
> *ds = DataSource(file_path, encoding='ISO-8859-1')layers = ds[0]*
>
> *#need something HERE to coerce geometry to 2D*
>
>
> *obj=MyModel(geometry=GEOSGeometry(layers[0].geom.hex))*
> *obj.save()*
>

[Jason]

I had a similar issue when uploading a kml to geojson conversion to postgis using geodjango.  There is no efficient way to do it in Django, so I ended up doing a workaround

• Create two geometry types, one with dims=2 and other with dims=3.  Allow both to be null and set default to null
• When updating, check the geometry dimension via GEOSGeometry.hasz.  If the return is 2, save the geom in the 2D field, otherwise save to the 3D

You can create a save/retrieve method on the model to handle this for you.

[a.gr...@gmail.com]

Jason: when you say create 2 geometry types one with 2 dimensions etc... how do you specify how many dimensions to have?

Because I created the field in this way: 

geom = models.MultiPolygonField(geography=True, spatial_index=True)

and let the Django migration do the job, but I don't know how to specify that it's a 3D field.

Thanks

[George Silva]

There is a way to coerce the input to 2d. It's not pretty, but it works.

from django.contrib.gis.geos.prototypes.io import wkt_w
wkt = wkt_w(dim=2).write(input_geom).decode()
geom = GEOSGeometry(wkt, srid=4674)

Imagine that you get input_geom from somewhere. In our case it was coming from a KML upload. We forced it to 2d using that snippet of code.

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[a.gr...@gmail.com]

Hi,

not sure if I did the right thing or not, but I just altered the table in this way https://github.com/alphagov/land-availability-api/pull/20/files#diff-f8cf5fd1898b907b7e7cc07dcc455602R15

(yes, the code is opensource!) because I've also noticed that GeoDjango doesn't have an equivalent of MultiPolygonZ so it must be created with a SQL command.

On Monday, 19 December 2016 15:35:50 UTC, george wrote:

There is a way to coerce the input to 2d. It's not pretty, but it works.

from django.contrib.gis.geos.prototypes.io import wkt_w
wkt = wkt_w(dim=2).write(input_geom).decode()
geom = GEOSGeometry(wkt, srid=4674)

Imagine that you get input_geom from somewhere. In our case it was coming from a KML upload. We forced it to 2d using that snippet of code.

On Mon, Dec 19, 2016 at 9:51 AM, <a.gr...@gmail.com> wrote:

Jason: when you say create 2 geometry types one with 2 dimensions etc... how do you specify how many dimensions to have?

Because I created the field in this way: 

geom = models.MultiPolygonField(geography=True, spatial_index=True)

and let the Django migration do the job, but I don't know how to specify that it's a 3D field.

Thanks

On Tuesday, 8 March 2016 16:34:35 UTC, Jason wrote:

I had a similar issue when uploading a kml to geojson conversion to postgis using geodjango.  There is no efficient way to do it in Django, so I ended up doing a workaround

• Create two geometry types, one with dims=2 and other with dims=3.  Allow both to be null and set default to null
• When updating, check the geometry dimension via GEOSGeometry.hasz.  If the return is 2, save the geom in the 2D field, otherwise save to the 3D

You can create a save/retrieve method on the model to handle this for you.

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[Rukaya Johaadien]

I did the following: 

from django.contrib.gis.geos import GEOSGeometry, WKTWriter

wkt_w = WKTWriter()

wkt_w.outdim = 2 # This sets the writer to output 2D WKT

polygon = GEOSGeometry(json.dumps(item['geometry'])) # The 3D geometry

temp = wkt_w.write(polygon)

polygon = GEOSGeometry(temp) # The 3D geometry

Here it is in the docs: https://docs.djangoproject.com/en/1.11/ref/contrib/gis/geos/#django.contrib.gis.geos.WKBWriter.outdim