python gdal tutorial: reading and writing vector data with ogr

Why use open source?

Advantages

1. Free, suitable for individuals and small companies

2. Powerful development tools, easier to find bugs

3. Cross-platform, can be used on windows and Linux

4. Stunning!

Disadvantages

1. No built-in geoprocessor

2. Few people use it

Open source RS/GIS module

1. OGR vector library: simple vector data reading and writing, is part of GDAL

2. GDAL geospatial data abstraction library:

a) Reading and writing raster data

b) ArcGIS is also developed based on GDAL

c) C++ library, but you can use python to call

related modules

1. Numeric: High-speed array processing is especially important for raster data

2. NumPy: the next generation of Numeric

3. A more powerful gis library http://www.gispython.org/

Import library:

import ogr

Or:

from osgeo import ogr

The universal method is:

try:

from osgeo import ogr

except:

import ogr

To read a certain type of data, it must be loaded first Data-driven means initializing an object so that it "knows" a certain data structure.

import ogr

driver = ogr.GetDriverByName('ESRI Shapefile')

The open() method of the data-driven driver returns a data source object

open(, )

where update 0 means read-only, 1 means writable. For example:

from osgeo import ogr

driver = ogr.GetDriverByName('ESRI Shapefile')

filename = 'C:/Users/gongwei/Documents/My eBooks /python_and_sage/GDAL python/test/ospy_data1/sites.shp'

dataSource = driver.Open(filename,0)

if dataSource is None:

print 'could not open'

sys.exit(1)

print 'done!'

Note that filename must be an absolute path!

Because absolute paths must be used, in order to simplify the code, os.chdir() is often used

to read the data layer

layer = dataSource.GetLayer(0)

Generally, ESRI shapefiles are filled with 0. If not filled in, the default is 0.

Look at how many points there are in this data layer?

n = layer.GetFeatureCount()

print 'feature count:', n

Read the upper, lower, left and right boundaries

extent = layer.GetExtent()

print 'extent:', extent

print 'ul: ', extent[0], extent[3]

print 'lr:', extent[1], extent[2]

Read a certain element feature (finally get to the point), what is read here is a point #should be a different id

print fid

In addition, read features in order and loop through all features

feat = layer.GetNextFeature() #Read the next

while feat:

feat = layer.GetNextFeature ()

later.ResetReading() #Reset

Extract the geometry of the feature

geom = feat.GetGeometryRef()

geom.GetX()

geom.GetY()

print geom.

Release memory

feature.Destroy()

Close the data source, which is equivalent to closing the file in file system operations

dataSource.Destroy()

Let’s talk about how to write it after reading it

Create a new file

driver.CreateDataSource(

But this file cannot already exist, otherwise an error will occur

Create a new layer

dataSource.CreateLayer(,CreateLayer(, geom_type=, [srs])

For example:

ds2 = driver.CreateDataSource('test.shp')

layer2 = ds2.CreateLayer('test', geom_type=ogr.wkbPoint)

To delete a shp file

driver.DeleteDataSource ('test.shp')

To add a new field, it can only be added in the layer, and there cannot be data.

If the added field is a string, the width must be set

fieldDefn = ogr.FieldDefn( 'id', ogr.OFTString)

fieldDefn.SetWidth(4)

layer.CreateField(fieldDefn)

To add a new feature, you must first complete the previous step and add all the fields

and then start from layer Read the corresponding feature type in and create feature

featureDefn = layer.GetLayerDefn()

feature = ogr.Feature(featureDefn)

Set the geometric shape

feature.SetGeometry(point)

Set a certain field The value of

feature.SetField('id', 23)

Write the feature into layer

layer.CreateFeature(feature)

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