[GIS] What does v.distance calculate exactly

distancegrasspolygonqgis

As I am from an economics department, I am quite a beginner in QGIS.
I read, that if I wanted to calculate the distance from a polygon to a point/line/another polygon, I should use the GRASS plugin v.distance.

Does anyone know how exactly the plugin calculates the distance? Let's say I want to know what's the minimum distance from Bavaria to the sea. Does v.distance take the center of Bavaria or does it divide the border into small points and then calculate the distance to the nearest sea spot?

Best Answer

Markus Metz, one of the GRASS developers, described in one of his posts how such features are calculated.

Here is the quote:

For lines to lines, say line A to line B, it calculates shortest distance of each vertex in A with each segment (not vertex) in B. So far, this is the same like in the original version, results are identical. Now if line A is a line or boundary, it calculates the shortest distance of each vertex in B to each segment in A. Additionally, it checks for intersections. In case of intersections, the first intersection found is used and the distance set to zero.

For lines to areas, it behaves similar to the original version. If a line is inside an area, the distance is set to zero. The first point of the line inside the area is used as common point. The distance is also set to zero if the line intersects with the outer ring or any of the inner rings (isles), in which case the fist intersection is used as common point.

For areas to areas, the module checks first for overlap or if one area is (partially) inside the other area. This is computationally quite intensive. If the outer rings of the two areas do not overlap, the distance is calculated as above for lines to lines, treating the outer rings as two lines. Again, the first point encountered falling into an area is used as common point, or the first intersection point.

For anything else than points to lines, there can be several common points with distance = zero, and the closest location could then be several points or several lines or several areas (result of overlay). For v.distance, I choose to select a single point, and not create an overlay like v.overlay.

Hope this clears it up a little!

Related Solutions

[GIS] How to calculate distances in a point sequence

I return to this issue because it is very similar to How do I find vector line bearing in QGIS or GRASS? and it can be solved with Python in the same way:

1) Haversine distance

One can find lots of scripts by searching Haversine distance with Python on the Internet and I choose one of them in Haversine Formula in Python (Bearing and Distance between two GPS points)

def haversine(lon1, lat1, lon2, lat2):
    """
    Calculate the great circle distance between two points 
    on the earth (specified in decimal degrees)
    """
    # convert decimal degrees to radians 
    lon1, lat1, lon2, lat2 = map(math.radians, [lon1, lat1, lon2, lat2])
    # haversine formula 
    dlon = lon2 - lon1 
    dlat = lat2 - lat1 
    a = math.sin(dlat/2)**2 + math.cos(lat1) * math.cos(lat2) * math.sin(dlon/2)**2
    c = 2 * math.asin(math.sqrt(a)) 
    km = 6367 * c
    return km

We have a series of lines (points) in the file that must be treated in pairs (point1 - point2) to calculate the distance. For this we will use a simple iterator from Most pythonic way to get the previous element

def offset(iterable):
    prev = None
    for elem in iterable:
        yield prev, elem
        prev = elem

Now it is possible to read the file (example of Kerrie) in pairs of lines/points

import csv
with open('testhavers.csv', 'rb') as f:
   reader = csv.DictReader(f)
   for  pair in offset(reader):
       print pair

 (None, {'LAT': '10.08527', 'LON': '124.59833', 'ID': '1', 'TIME': '21:24:37'})
 ({'LAT': '10.08527', 'LON': '124.59833', 'ID': '1', 'TIME': '21:24:37'},
 {'LAT':    '10.08523', 'LON': '124.59830', 'ID': '2', 'TIME': '21:25:07'})
 ({'LAT': '10.08523', 'LON': '124.59830', 'ID': '2', 'TIME': '21:25:07'}, 
 {'LAT': '10.08526', 'LON': '124.59832', 'ID': '3', 'TIME': '21:25:37'})
 ({'LAT': '10.08526', 'LON': '124.59832', 'ID': '3', 'TIME': '21:25:37'}, 
 {'LAT':    '10.08526', 'LON': '124.59831', 'ID': '4', 'TIME': '21:26:07'})

Then create a shapefile containing the original fields of the csv file and a new field for the distance with the Python modules Shapely and Fiona of Sean Gillies:

import fiona
from shapely.geometry import Point, mapping
# creation of the schema of the shapefile (geometry and fields)
schema = { 'geometry': 'Point', 'properties':{'ID': 'int', 'LAT':'float', 'LON':'float', 'TIME':'str','distance' : 'float'}}
# creation of the shapefile:
with fiona.collection("result.shp", "w", "ESRI Shapefile", schema) as output:
    # reading the csv file
    with open('testhavers.csv', 'rb') as f:
       reader = csv.DictReader(f)
       # we need here to eliminate the first pair of point with None
       for i, pair in enumerate(offset(reader)):
            if i == 0: (pair with None)
                # writing of the point geometry and the attributes
                point = Point(float(pair[1]['LON']), float(pair[1]['LAT']))
                dist = 0 # None
                output.write({'properties': {'ID':int(pair[1]['ID']),'LAT':float(pair[1]['LAT']),'LON':float(pair[1]['LON']), 'TIME':pair[1]['TIME'],'distance': dist},'geometry': mapping(point)})
             else:
                # writing of the point geometry and the attributes
                point = Point(float(pair[1]['LON']), float(pair[1]['LAT']))
                # Haversine distance between pairs of points
                dist = haversine(float(pair[0]['LON']), float(pair[0]['LAT']), float(pair[1]['LON']),float(pair[1]['LAT']))
                output.write({'properties': {'ID':int(pair[1]['ID']),'LAT':float(pair[1]['LAT']),'LON':float(pair[1]['LON']), 'TIME':pair[1]['TIME'],'distance': dist},'geometry': mapping(point)})

and the result: enter image description here

It is also possible to do it with PyQGIS but it is more complex than Fiona which uses simple dictionaries to create shapefiles.

You can use another function to calculate the Haversine distance (Why is law of cosines more preferable than haversine when calculating distance between two latitude-longitude points?) without any problem, only the distance calculation changes, not the process of creating the shapefile.

Minimum Distance Calculation – From Polygon Centroid to Edge Using ArcGIS Desktop and ModelBuilder

The following approach requires you to have an Advance license:

Convert your polygons to lines using the Feature To Line tool
Identify the distance from your centroid point to the line using the Generate Near Table tool

These two steps can be wrapped up in a model.

Best Answer

Related Solutions

[GIS] How to calculate distances in a point sequence

Minimum Distance Calculation – From Polygon Centroid to Edge Using ArcGIS Desktop and ModelBuilder

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