Geodesic Lines to GIS

Computes geodesic lines from start point to end point and stores them in a GIS file (Shapefile and GeoJSON). A geodesic is the shortest path between two points on a curved surface, like an ellipsoid of revolution (Read more on Wikipedia).

This code is builded on top of three libraries: Pyproj, Fiona and Shapely.

There are several libraries to compute geodesic distances solving the geodesic inverse problem (to find the shortest path between two given points). I chose Pyproj because it works fine for this purpose and is an interface to a widely used library in the geospatial industry (Proj4 C library). Actually Proj4 C library (>= v.4.9.0) routines used to compute geodesic distance are a simple transcription from excellent Geographiclib C++ Library developed by Charles Karney. Proj4 C library < v.4.9.0 uses Paul D. Thomas algorithms. You can see more about this here: GeodeticMusings: a little benchmark of three Python libraries to compute geodesic distances.

All computations are performed with WGS84 ellipsoid and the CRS (Coordinate Reference System) of GIS file outputs are EPSG:4326.

In the examples section you can see the problem of calculating lines crossing antimeridian is solved.

Numpy array is supported as inputa data.

Read more on:

[Link to Geographica Blog] (http://www.blog-geographica.com/2015/06/11/geodesic-lines-gis/)

Installing

You can install this package from PYPI: https://pypi.python.org/pypi/GeodesicLinesToGIS

$ pip install GeodesicLinesToGIS

Geodesic lines examples

Below are shown different geodesic lines computed with this library on several map projections. Also you can see the relation with rhumb lines (loxodromic) and straight lines between the same points.

• Geodesic line (computed): green.
• Loxodromic: red.
• Straight line: dashed black.

Maximun differences occur between Mercator (loxodromic is a straight line) and Gnomonic projection (geodesic is a straight line).

[Data and maps are here] (https://github.com/GeographicaGS/GeodesicLinesToGIS/tree/master/data)

Mercator projection - Proj4 string: '+proj=merc +lon_0=0 +k=1 +x_0=0 +y_0=0 +datum=WGS84 +units=m +no_defs'

Gnomonic projection (centered: 50W and 60N) Proj4 string: '+proj=gnom +lat_0=60 +lon_0=-50 +x_0=0 +y_0=0 +datum=WGS84 +units=m +no_defs'

Azimuthal Equidistant projection (centered: 50W and 30N) - Proj4 string: '+proj=aeqd +lat_0=30 +lon_0=-50 +x_0=0 +y_0=0 +datum=WGS84 +units=m +no_defs'

Lambert Azimuthal Equal Area projection (centered: 50W and 60N) Proj4 string: '+proj=laea +lat_0=60 +lon_0=-50 +x_0=0 +y_0=0 +datum=WGS84 +units=m +no_defs'

Antimeridian problem solved

You can see the problem of calculating lines crossing antimeridian is solved.

Mercator projection - Proj4 string: '+proj=merc +lon_0=0 +k=1 +x_0=0 +y_0=0 +datum=WGS84 +units=m +no_defs'

Mercator projection (centered: 150E) - Proj4 string: '+proj=merc +lon_0=150 +k=1 +x_0=0 +y_0=0 +datum=WGS84 +units=m +no_defs'

Requirements

• Pyproj, https://github.com/jswhit/pyproj
• Fiona, https://github.com/Toblerity/Fiona
• Shapely, https://github.com/Toblerity/Shapely

Usage

Usage is very simple. There are two modes:

• Single input (one start/end).
• Multiple input (more than one start/end).

Single input

Single input usage.

from geodesiclinestogis import GeodesicLine2Gisfile

lons_lats: input coordinates. (start longitude, start latitude, end longitude, end latitude)

lons_lats = (-3.6,40.5,-118.4,33.9)

Folder path to store output file and filename:

folderpath = '/tmp'

layername = "geodesicline"

Create object. You can pass two parameters:

• antimeridian: [True | False] to solve antimeridian problem (default is True).
• prints: [True | False] print output messages (default is True).

gtg = GeodesicLine2Gisfile()

Launch computations. You can pass two parameter:

• lons_lats: input coords returned by gcComp.
• km_pts: compute one point each n km (default is 20 km)

cd = gtg.gdlComp(lons_lats, km_pts=30)

Dump geodetic line coords to Linestring Feature and store in a GIS file.

Output formats: "ESRI Shapefile" (default), "GeoJSON"

# shapefile output
gtg.gdlToGisFile(cd, folderpath, layername)

# geojson output
gtg.gdlToGisFile(cd, folderpath, layername, fmt="GeoJSON")

Multiple input

Multiple input usage.

from geodesiclinestogis import GeodesicLine2Gisfile

data = [
        (-6.,37.,-145.,11.),
        (-150.,37.,140.,11.),
        (-6.,37.,120.,50.),
        (-3.6,40.5,-118.4,33.9),
        (-118.4,33.9,139.8,35.5),
        (-118.4,33.9,104.,1.35),
        (-118.4,33.9,151.,-33.9),
        (-20.4,33.9,178.,-33.9)
    ]

folderpath = "/tmp/geod_line_mu"

layername = "geodesicline"

gtg = GeodesicLine2Gisfile()

gtg.gdlToGisFileMulti(data, folderpath, layername)

Numpy array (multiple) input

Numpy array input usage.

import numpy as np
from geodesiclinestogis import GeodesicLine2Gisfile

data_ = [
        (-6.,37.,-145.,11.),
        (-150.,37.,140.,11.),
        (-6.,37.,120.,50.),
        (-3.6,40.5,-118.4,33.9),
        (-118.4,33.9,139.8,35.5),
        (-118.4,33.9,104.,1.35),
        (-118.4,33.9,151.,-33.9),
        (-20.4,33.9,178.,-33.9)
    ]

data = np.array(data_)

folderpath = "/tmp/geod_line_np"

layername = "geodesicline"

gtg = GeodesicLine2Gisfile()

gtg.gdlToGisFileMulti(data, folderpath, layername)

About author

Developed by Cayetano Benavent. GIS Analyst at Geographica.

http://www.geographica.gs

License

This program is free software; you can redistribute it and/or modify it under the terms of the GNU General Public License as published by the Free Software Foundation; either version 2 of the License, or (at your option) any later version.

Third-Party licenses

You can read Pyproj, Fiona and Shapely licenses in the next links: https://raw.githubusercontent.com/jswhit/pyproj/master/LICENSE https://raw.githubusercontent.com/Toblerity/Shapely/master/LICENSE.txt https://raw.githubusercontent.com/Toblerity/Fiona/master/LICENSE.txt