Geographic Positionning

All geographic position information are manage via a GeoPosSet class.

You can open a file containing the geographic coordinates of the geophysical survey corresponding to a DataSet object from both :

  • an ascii file** (.csv for instance)

  • or a shapefile (.shp)

Reading positions from file

To open your a GeoPosSet from a file (ASCII or shapefile) use:

>>> from geophpy.geoposset import GeoPosSet

>>> # file type determined from extension
>>> gpos = GeoPosSet.from_file('GCPs.txt')

>>> # file type forced manually
>>> gpos = GeoPosSet.from_file('GCPs.shp', filetype='shapefile', )

Retrieve position

Retrieve list of Ground Control Points via:

>>> gcp_list = gpos.points_list
>>> # or
>>> gcp_list = gpos.GCPs()

>>> print(gcp_list)
>>> [[0, 32.52, 34.70], [1, 32.52, 34.70]]  # with [num, x or lon, y or lat]

Plot GCPs

Plot the Ground Control Points:

>>> fig = gpos.plot(long_label=False)
>>> fig.show()
_images/figGps1.png

Saving GCPs

Ground Control Points can be saved as an ascii file or a kml file:

>>> # saving as an ascii file
>>> gpos.to_ascii("MyGCPs.dat", delimiter=',')

To be saved as a kml file, the coordinates must be converted into lat,long first. If the original geographic system is UTM you can directly do:

>>> # Converting UTM to WGS84
>>> import geophpy.geoposset as pset
>>> num, east, north, xlocal, ylocal = gpos.GCPs().T
>>> lt, lg = pset.utm_to_wgs84(east, north, 32,'T')
>>> gcp_wgs84 = np.stack((num, xlocal, ylocal, lt, lg)).T

>>> # saving as kml
>>> gpos_conv = pset.GeoPosSet(refsystem='WGS84', points_list=gcp_wgs84)
>>> gpos.to_kml("ConvGCPs.kml")
_images/figGps2.png

Dataset georeferencing

Dataset georeferencing is possible with at less 4 points.

>>> # Georeferencing ungridded values
>>> dataset.setgeoref('UTM', gpos.points_list, 'T', 32)

>>> # Georeferencing gridded values
>>> dataset.interpolate(x_step=0.5, y_step=0.25)
>>> dataset.setgeoref('UTM', gpos.points_list, 'T', 32)

Plotting georeferenced data position (example for ungridded values)

>>> import geophpy.dataset as dset
>>> import geophpy.geoposset as pset

>>> # Reading files
>>> success, dataset = dset.DataSet.from_file('Mag_ex1.dat')
>>> success, gpos = pset.GeoPosSet.from_file('GPS_ex1.csv')

>>> # Displaying GCps
>>> gpos.plot(long_label=True)
_images/figGeorefGCPs.png

Fig. 85 Georeferencing - Ground Control Points.

 
>>> # Georeferencing
>>> dataset.setgeoref(gpos.refsystem, gpos.points_list, 'T', 32)

>>> # Displaying in local system
>>> box = np.stack(dataset.get_boundingbox())  # bounding box corners
>>> x, y = dataset.get_xyvalues()  # data sample position

>>> box_gcp = gpos.points_list.T[1:3].T  # georef bounding box corners
>>>  east, north = dataset.get_georef_xyvalues() # data sample georef position

>>> fig = plt.figure()
>>> ax = fig.add_subplot(111)
>>> ax.plot(x, y, 'r.', markersize=0.25) # or east, north
>>> ax.plot(box.T[0], box.T[1], 'bo') # or box_gcp.T[0], box_gcp.T[1]
>>> ax.set_aspect('equal')

>>> # adding GCPs label
>>> for point in  box:
>>>     strg = '(%s, %s)' % (point[0], point[1])
>>>     dx, dy = 3, 0
>>>     plt.text(point[0]+dx, point[1]+dy, strg,
>>>              bbox=dict(fc='white',ec='none', alpha=0.5))
_images/figGeorefLocalSystem.png

Fig. 86 Georeferencing - Dataset in local system.
_images/figGeorefRefSystem.png

Fig. 87 Georeferencing - Dataset in geographic system.

With the data set georeferenced, it is possible to export the dataset as a kml file:

>>> dataset.to_kml('2D-SURFACE', 'gray_r', "prospection.kml",
    cmmin=-10, cmmax=10, dpi=600)
_images/figGeoref1.png

Exporting the data set as a raster in a SIG application (as ArcGis, QGis, Grass, …) is possible with severals picture file format (‘jpg’, ‘png’, ‘tiff’):

>>> dataset.to_raster('2D-SURFACE', 'gray_r', "prospection.png",
    cmmin=-10, cmmax=10, dpi=600)
_images/figGeoref2.png

A world file containing positioning informations of the raster is created (‘jgw’ for JPG, ‘pgw’ dor.png, and ‘tfw’ for TIFF picture format) with:

Line 1: A: pixel size in the x-direction in map units/pixel

Line 2: D: rotation about y-axis

Line 3: B: rotation about x-axis

Line 4: E: pixel size in the y-direction in map units, almost always negative[3]

Line 5: C: x-coordinate of the center of the upper left pixel

Line 6: F: y-coordinate of the center of the upper left pixel

Example:

0.0062202177595

-0.0190627320737

0.0131914192417

0.00860610262817

660197.8178

3599813.97056

ASCII GCPs file format

If the geographic reference system is known, it must be written on the first line. The other lines contain the point number followed by its GPS coordinates (Longitude and Latitude or Easting and Northing) and eventually the corresponding local X, Y -coordinates.

>>> # WGS84 file without local coordinates
>>> # (delimiter is tabulation)
WGS84
1   66.84617533     37.74956917
2   66.84649517     37.7489535
3   66.8472475      37.74972867
4   66.84689417     37.7491385
5   66.84691867     37.7491025
...

>>> # UTM file can also contain the corresponding local coordinates
>>> # (delimiter is tabulation)
UTM
1   745038.191      4656005.727     150     0
2   745068.172      4656045.663     150     50
3   745028.43       4656076.057     100     50
4   744988.466      4656105.978     50      50
5   744998.428      4656036.093     100     0
...

>>> # UTM file with zone letter and number
>>> # (delimiter is tabulation)
UTM         L       32
1   745038.191      4656005.727     150     0
2   745068.172      4656045.663     150     50
3   745028.43       4656076.057     100     50
4   744988.466      4656105.978     50      50
5   744998.428      4656036.093     100     0
...

>>> # Unknown geographic system with missing local position
>>> # (delimiter is ";")
1;745038.191;4656005.727;150;0
2;745068.172;4656045.663;150;50
3;745028.43;4656076.057;;        # missing local position
4;744988.466;4656105.978;50;50
5;744998.428;4656036.093;100;0
...