I am trying to learn how to use ogr in python using the country and populated places datasets from http://www.naturalearthdata.com/downloads/50m-cultural-vectors/. I am attempting to use filters and buffers to find points (ne_50m_populated_places.shp) within a specified buffer of a named country (filtered from feature class ADMIN in ne_50m_admin_0_countries.shp). The script runs but returns populated places from around the Carribean region. The problem appears to be that I don't understand what units to use for buffer(). In the script I have simply used an arbitrary value of 10 to test if the script works. Ideally, I want to be able to specify a buffer distance, say 500km, but can't work out how to do this as my understanding is buffer() is using the units of countries.shp that will be in wgs84 lat/long format. Advice on a better method to achieve this would be much appreciated.

# import modules
import ogr, os, sys


## data source
os.chdir('C:/data/naturalearth/50m_cultural')

# get the shapefile driver
driver = ogr.GetDriverByName('ESRI Shapefile')

# open ne_50m_admin_0_countries.shp and get the layer
admin = driver.Open('ne_50m_admin_0_countries.shp')
if admin is None:
  print 'Could not open ne_50m_admin_0_countries.shp'
  sys.exit(1)
adminLayer = admin.GetLayer()

# open ne_50m_populated_places.shp and get the layer
pop = driver.Open('ne_50m_populated_places.shp')
if pop is None:
  print 'could not open ne_50m_populated_places.shp'
  sys.exit(1)
popLayer = pop.GetLayer()

# use an attribute filter to restrict ne_50m_admin_0_countries.shp to "Angola"
adminLayer.SetAttributeFilter("ADMIN = ANGOLA")

# get the Angola geometry and buffer it by 10 units
adminFeature = adminLayer.GetFeature(0)
angolaGeom = adminFeature.GetGeometryRef()
bufferGeom = angolaGeom.Buffer(10)

# use bufferGeom as a spatial filter on ne_50m_populated_places.shp to get all places
# within 10 units of Angola
popLayer.SetSpatialFilter(bufferGeom)

# loop through the remaining features in ne_50m_populated_places.shp and print their
# id values
popFeature = popLayer.GetNextFeature()
while popFeature:
  print popFeature.GetField('NAME')
  popFeature.Destroy()
  popFeature = popLayer.GetNextFeature()
  
# close the shapefiles
admin.Destroy()
pop.Destroy()