Making buffer from line using GDAL and Python

Question

I am trying to make a buffer from a line using gdal.

I use the example in the gdal Cookbook

import ogr, os
def createBuffer(inputfn, outputBufferfn, bufferDist):
 inputds = ogr.Open(inputfn)
 inputlyr = inputds.GetLayer()
 shpdriver = ogr.GetDriverByName('ESRI Shapefile')
 if os.path.exists(outputBufferfn):
 shpdriver.DeleteDataSource(outputBufferfn)
 outputBufferds = shpdriver.CreateDataSource(outputBufferfn)
 bufferlyr = outputBufferds.CreateLayer(outputBufferfn, geom_type=ogr.wkbPolygon)
 featureDefn = bufferlyr.GetLayerDefn()
 for feature in inputlyr:
 ingeom = feature.GetGeometryRef()
 geomBuffer = ingeom.Buffer(bufferDist)
 outFeature = ogr.Feature(featureDefn)
 outFeature.SetGeometry(geomBuffer)
 bufferlyr.CreateFeature(outFeature)
 def main(inputfn, outputBufferfn, bufferDist):
 createBuffer(inputfn, outputBufferfn, bufferDist)
 if __name__ == "__main__":
 inputfn = 'obli.shp'
 outputBufferfn = 'oblibuffer001.shp'
 bufferDist = 0.01
 main(inputfn, outputBufferfn, bufferDist)
 # creating the projection file. Not included in the cookbook
 prj = open('oblibuffer001.prj', 'w')
 proyeccion = 'GEOGCS["GCS_WGS_1984",
 DATUM["D_WGS_1984",SPHEROID["WGS_1984",6378137.0,298.257223563]],' \
 'PRIMEM["Greenwich",0.0],UNIT["Degree",0.0174532925199433]]'
 prj.write(proyeccion)
 prj.close()

It works fine

http://pcjericks.github.io/py-gdalogr-cookbook/vector_layers.html?highlight=buffer

But I can not understand exactly what kind of units I must use in the bufferDist

Ussing 0.01 I think it was a kilometre but the result is different depending the direction of the line. For example Using a parallel_direction_line using bufferDist = 0.01 meassured in the map the buffer is 1,1 km a meridiam_direction_line using bufferDist = 0.01 meassured in the map the buffer is 0.456 km

but a oblicual_direction_line using bufferDist = 0.01 meassured in the map the buffer is 0,57 km

How can I find the value for the bufferDist variable I think depends on the latitude and the direction of the line?

Answer 1

Your buffer distance is given in the units of your dataset. Your coordinate system is lon/lat geographic. Thus 0.01 = 0.01 decimal degrees. To be able to buffer by 1 km (1000 m) you need to reproject your dataset to a projected coordinate system which has units in metres.

There are examples in the Python GDAL/OGR Cookbook of how to reproject layers and geometries.

• kamome: Then, Is it posible to buffer ussing milles ? or is a stupid thing
• kamome: Then, Is it posible to buffer using miles ? or is a stupid thing. 1º = 60' then 1 mille = 0.00166 in the buffer. I am working with some shapes in WGS84 without protection, then I try to maintain the whole shapes without project. What do you think about
• user2856: You can buffer in miles if you reproject your layer to a coordinate system that uses metres (1 mile = 1609.34 m) or feet (1 mile = 5280 ft) as the units.