[gdal-dev] [python-ogr] Transform datasource problems

[Frank Broniewski]

Even,

thank you for your verbose and helpful answer!

Am 09.02.2011 20:24, schrieb Even Rouault:
> Several points :
>
> 1) The implementation of Layer.GetFeature() is only efficient for just a few
> drivers (shapefile). Other drivers will need to sequentially read from the first
> feature ... So use GetNextFeature()

Are these informations somewhere documented? I had a look at the docs 
here [1],[2], but didn't find anything related. What about iterating 
over layers in a datasource, or fields in a feature? Are there similar 
concerns?

> 2) The real bug is that you don't rewrite the feature to its layer. So you
> need to add a target_layer.SetFeature(target_feature)

Ah, Ok, so the link/object relation between the layer and the feature is 
lost, once the feature is fetched? Since most things are done by 
reference in Python, this was not obvious to me. Thanks for pointing 
this out!

> 3) But what you are doing is really what ogr2ogr.py does, although your
> approach will be less efficient as you first copy the layer and then rewrite each
> of its feature.

Yes, I know. And I had a deep look at the ogr2ogr.py script before 
writing to the list, but frankly this is too much for me. It looks more 
like a c/c++ script than anything else to me - no functions, no classes, 
nothing from the python zen ... I really got lost in the middle of the 
script :-) but that's just a cause of my insufficient programming skills.

> For best performance, you should rather :
> a) create a target layer from scratch
> b) copy the feature definition from the source layer to the target layer
> c) iterate over the source features, for each source feature, create a target
> feature (Feature.SetFrom(other_feature)). reproject the geometry, assign it to
> the target feature, and Layer.CreateFeature(target_feature)
>
> Basically a stripped version of ogr2ogr.py

I will make a new version and post it here on the list, so everyone can 
have a look and maybe learn something from it


Frank


[1] http://www.gdal.org/ogr/ogr_formats.html
[2] http://www.gdal.org/ogr/classOGRLayer.html

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Frank BRONIEWSKI

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