Shapefile – Adding Custom Feature Attributes Using OGR in Python

ogrosgeoshapefile

I am seeking a way to take an existing Shapefile that has a Feature set of 200 countries. Each country Feature has an attribute of "NAME." My objective is to create a Python script that adds an arbitrary (for now) additional attribute, say, "POPULATION".

Of course I have the OSGeo and GeoDjango modules installed. I'm as far as:

 from osgeo import ogr

    infile = ogr.Open('sample.shp', 1) #'sample.shp' is a pre-existing ESRI shapefile described above
    inlyr = ogr.GetLayerByIndex(0)

Am I missing an OGR function that will allow me to insert Feature attribute fields into an existing Shapefile?

Best Answer

Is it possible to add a field to an existing shapefile using Python OGR..

from osgeo import ogr
driver = ogr.GetDriverByName('ESRI Shapefile')
dataSource = driver.Open(“c:/test/Test2.shp”, 1) #1 is read/write

#define floating point field named DistFld and 16-character string field named Name:
fldDef = ogr.FieldDefn('DistFld', ogr.OFTReal)
fldDef2 = ogr.FieldDefn('Name', ogr.OFTString)
fldDef2.SetWidth(16) #16 char string width

#get layer and add the 2 fields:
layer = dataSource.GetLayer()
layer.CreateField(fldDef)
layer.CreateField(fldDef2)

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I suspect it's because the underlying GEOS library only work in Cartesian space rather than spherical, so you'll have to subtract 360 from any longitudinal coordinate greater than 180, which makes 359 == -1. Of course, you'll still have problems with features crossing the anti-meridian (i.e. +- 180 degrees longitude), but you can easily detect that and not do the 360 subtraction step.

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It is a pure Python problem and not an GDAL/OGR problem. But if you want to program in Python,you need to understand the difference between an iterator and an iterable (many, many references on the Web, but one of the best is Loop Like A Native by Ned Batchelder). These are fundamental concepts in Python.

"The important operation on an iterable is iter(), which will return an iterator. And the only operation available on an iterator is next(), which either returns the next value, or raises StopIteration, a special exception that means the iteration is finished." (from Ned Batchelder)

An iterable produces an iterator and an iterator produces a stream of values.

So what's the difference between the for loop and pointsLayer.GetNextFeature() ?

the for loops = iterator and pointsLayer= iterable

for feature in iterable:
   statements

so with your example:

from osgeo import ogr
source = ogr.Open('yourpointshapefile.shp')
pointsLayer = source.GetLayer()
for feature in pointsLayer:
    geom =feature.GetGeometryRef()
    xy = geom.GetPoint()
    print xy

(272022.68669955182, 155404.12013724342, 0.0)
(272904.99338241993, 152881.6706538822, 0.0)
.....
(272796.14718989708, 152075.00336062044, 0.0)

But we can create another type of iterator with iter() and next():

source = ogr.Open('yourpointshapefile.shp')
pointsLayer = source.GetLayer()
iterator = iter(pointsLayer)
# first feature in pointsLayer
feature = iterator.next()
geom = feature.GetGeometryRef()
xy = geom.GetPoint()
print xy
(272022.68669955182, 155404.12013724342, 0.0)
# second feature in pointsLayer
feature = iterator.next()
geom = feature.GetGeometryRef()
xy = geom.GetPoint()
print xy
(272904.99338241993, 152881.6706538822, 0.0)
....
# end raises StopIteration error to signal that iteration is complete
feature = iterator.next()
Traceback (most recent call last):
...
StopIteration

Unlike the for loop that handles the StopIteration error, you need here another control structure (while loop,if,...)

Thus, what is pointsLayer.GetNextFeature() ?

It is an iterator and you can replace iterator = iter(pointsLayer) and iterator.next() by

feature = pointsLayers.GetNextFeature()
geom = feature.GetGeometryRef()
xy = geom.GetPoint()
.....
feature = pointsLayers.GetNextFeature()
....

I hope that I have been able to explain why you should not mix the for loops (iterator) with .GetNextFeature() (another iterator).

Best Answer

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