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@tmcw /0README.md
Last active Feb 9, 2017

What would you like to do?

redo: Combine Several Fields into One.

A sort of continuation of GIS with Python, Shapely, and Fiona.

Combine several fields into one with Python, Shapely, and Fiona! You'll also want GDAL around.

First, let's start out with data. I'll use Natural Earth Countries 1:50m. Download that file and move it to a directory.

This is assuming you're using a terminal. If you have a mac, that's the Terminal.app.

Unzip that file. Then find out about its fields with ogrinfo. -al indicates all layers, and -so means show a summary only - don't show every individual feature in the file.

$ ogrinfo -so -al ne_50m_admin_0_countries.shp
INFO: Open of `ne_50m_admin_0_countries.shp'
      using driver `ESRI Shapefile' successful.
Layer name: ne_50m_admin_0_countries
Geometry: 3D Polygon
Feature Count: 241
Extent: (-180.000000, -89.998926) - (180.000000, 83.599609)
Layer SRS WKT:
GEOGCS["GCS_WGS_1984",
    DATUM["WGS_1984",
        SPHEROID["WGS_84",6378137.0,298.257223563]],
    PRIMEM["Greenwich",0.0],
    UNIT["Degree",0.0174532925199433]]
scalerank: Integer (4.0)
featurecla: String (30.0)
labelrank: Real (16.6)
sovereignt: String (254.0)
sov_a3: String (254.0)
adm0_dif: Real (16.6)
level: Real (16.6)
type: String (254.0)
admin: String (254.0)
adm0_a3: String (254.0)
# truncated, there's lots more

First let's get comfortable with the layout of Fiona - check out copy.py.

Once you're comfortable there, check out combining_fields.py. It's only two lines different than copy.py - the first is

schema['properties']['a3_sov'] = 'str'

Which says that in the new schema (layout, or database format), there's a new field called a3_sov and it's a string.

Then, on the output loop,

poly['properties']['a3_sov'] = poly['properties']['sov_a3'] + poly['properties']['sovereignt']

Creates this new field by pulling the sov_a3 and sovereignt fields and combining them with +. This will create a new file called ne_50m_admin_0_countries_a3_sov.shp.

See Also

from shapely.geometry import mapping, shape
from fiona import collection
with collection('ne_50m_admin_0_countries.shp', 'r') as input:
schema = input.schema.copy()
# declare that the new shapefile will have
# a new field called a3_sov, which is a string.
schema['properties']['a3_sov'] = 'str'
with collection(
'ne_50m_admin_0_countries_a3_sov.shp', 'w', 'ESRI Shapefile', schema) as output:
for poly in input:
# create a new field
poly['properties']['a3_sov'] = poly['properties']['sov_a3'] + poly['properties']['sovereignt']
output.write({
'properties': poly['properties'],
'geometry': mapping(shape(poly['geometry']))
})
from shapely.geometry import mapping, shape
from fiona import collection
# Read the Shapefile
with collection('ne_50m_admin_0_countries.shp', 'r') as input:
# The output has the same schema
schema = input.schema.copy()
# write a new shapefile
with collection(
'ne_50m_admin_0_countries_copy.shp', 'w', 'ESRI Shapefile', schema) as output:
for poly in input:
output.write({
'properties': poly['properties'],
'geometry': mapping(shape(poly['geometry']))
})

GDAL (provides ogrinfo)

Option 1:

Install Homebrew. It's a useful tool for installing all kinds of open source tools on Macs.

brew install gdal

Option 2:

Download a Kyngchaos build of GDAL.

pip

sudo easy_install pip

Fiona & Shapely

pip install Fiona
pip install Shapely

tbonfort commented Mar 7, 2013

afaik, shapefiles do not contain multiple layers, it's OGR that adds this abstraction in order for it to support datasources with multiple layers (e.g. multiple tables in a postgis database). You can replace your first commands with

ogrinfo -so -al FILENAME.shp

which lists the info of all (i.e. the only) the shapefile's layers

mlaloux commented Mar 7, 2013

You do not need homebrew if you install gdal from http://www.kyngchaos.com/software/frameworks

Owner

tmcw commented Mar 7, 2013

@mlaloux - thanks, added a note

@tbonfort - added a correction

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