How to access vector coordinates in GRASS GIS from python?.md

From How to access vector coordinates in GRASS GIS from python?

For walking a graph you have all the v.net family. To extract the vertices coordinates of lines or polygons (and other columns) one solution is, as always, through v.out.ascii that gives you the xy(z) coordinates(see How one can find the starting and end point of a line in a vector file or how one can connect two line end to end using C code)

    test = grass.read_command("v.out.ascii", input="yourvector", format="standard")

The result is given in v.out.ascii. In my case, for example, the variable test is:

ORGANIZATION: 
DIGIT DATE: 
MAP NAME:  
MAP DATE:     Sun Jun 24 10:16:35 2012
MAP SCALE:    1
OTHER INFO:  
ZONE:         0
MAP THRESH:   0.000000
VERTI:
L  7 1
206643.21517601 125181.18058576
201007.33432923 121517.8555206
208615.77587567 118699.91687157
199034.77765775 115036.59058769
200725.54321492 111936.8560102
192835.30987687 107428.14794157
192835.30987687 107428.14794157
1 1

It is a (poly)line with one element and 7 vertices (L 7 1) and test is a Python string so

    result = test.split("\n") 

and you have a list with the lines of test. If you use regular expressions:

    for line in result:
    if re.findall(r'^.[0-9]+\.',line):
        print line

     206643.21517601 125181.18058576
     201007.33432923 121517.8555206
     208615.77587567 118699.91687157
     199034.77765775 115036.59058769
     200725.54321492 111936.8560102
     192835.30987687 107428.14794157
     192835.30987687 107428.14794157

and

    coords = []    
    for line in result:
       if re.findall(r'^.[0-9]+\.',line): 
           coords.append(line.strip().split(" "))

gives you the solution

    [['206643.21517601', '125181.18058576'], ['201007.33432923', '121517.8555206'], ['208615.77587567', '118699.91687157'], ['199034.77765775', '115036.59058769'], ['200725.54321492', '111936.8560102'], ['192835.30987687', '107428.14794157'], ['192835.30987687', '107428.14794157'], ['206643.21517601', '125181.18058576'], ['201007.33432923', '121517.8555206'], ['208615.77587567', '118699.91687157'], ['199034.77765775', '115036.59058769'], ['200725.54321492', '111936.8560102'], ['192835.30987687', '107428.14794157'], ['192835.30987687', '107428.14794157']]

After that, you can create a new point vector file with the xy values and How one can find the starting and end point of a line in a vector file or how one can connect two line end to end using C code

If you want to use OGR python bindings, it is easy

    from osgeo import ogr
    # open grass vector
    ds = ogr.Open('/grassdata/geol/test/vector/testline/head')
    layer = ds.GetLayer(0)
    layer.GetName()
    'testline'
     feat = layer.GetFeature(0)
     geom = feature.GetGeometryRef()
     geom.ExportToWkt()
     'LINESTRING (206643.21517600651714 125181.180585758760571,201007.334329231875017 121517.855520597659051,208615.775875667924993 118699.916871574707329,199034.77765774706495 115036.59058768954128,200725.543214922072366 111936.85601020231843,192835.30987687263405 107428.147941565141082,192835.30987687263405 107428.147941565141082)'
     for i in range(geometry.GetPointCount()):
         xy = geometry.GetPoint(i)
         print xy
 
    (206643.21517600652, 125181.18058575876, 0.0)
    (201007.33432923188, 121517.85552059766, 0.0)
    (208615.77587566792, 118699.91687157471, 0.0)
    (199034.77765774706, 115036.59058768954, 0.0)
    (200725.54321492207, 111936.85601020232, 0.0)
    (192835.30987687263, 107428.14794156514, 0.0)
    (192835.30987687263, 107428.14794156514, 0.0)

After that you can use other modules like shapely

    from shapely.wkt import loads
    line =loads(geometry.ExportToWkt())
    list(line.coords) 
    [(206643.21517600652, 125181.18058575876), (201007.33432923188, 121517.85552059766), (208615.77587566792, 118699.91687157471), (199034.77765774706, 115036.59058768954), (200725.54321492207, 111936.85601020232), (192835.30987687263, 107428.14794156514), (192835.30987687263, 107428.14794156514)]