Python grass

Python scripting in GRASS GIS environment

Geospatial Analysis and Modeling – MEA792
Margherita Di Leo


Python + GRASS GIS:

➢ Python scripts that call GRASS functionality from
outside

➢ Running external commands from Python (as a
grass module)


Example

Module: r.ipso

Purpose: Creates the ipsographic curve and the
adimensional ipsometric curve

Requires: Matplotlib

http://svn.osgeo.org/grass/grass-addons/raster/r.ipso/


GUI
#%module
#% description: Creates the ipsographic and ipsometric curve
#% keywords: raster
#%end
#%option
#% key: map
#% type: string
#% gisprompt: old,raster,raster
#% key_desc: name
#% description: Name of elevation raster map
#% required: yes
#%end
#%option
#% key: image
#% type: string
#% gisprompt: new_file,file,input
#% key_desc: image
#% description: output graph
#% required: yes
#%end
#%flag
#% key: a
#% description: generate ipsometric curve
#%END
#%flag
#% key: b
#% description: generate ipsographic curve
#%END


prompt
GRASS 6.5.svn (Basilicata):~ > r.ipso.py --help

Description:
Creates the ipsographic and ipsometric curve

Keywords:
raster

Usage:
r.ipso.py [-ab] map=name image=image [--verbose] [--quiet]

Flags:
-a generate ipsometric curve
-b generate ipsographic curve
--v Verbose module output
--q Quiet module output

Parameters:
map Name of elevation raster map
image path to output graph
GRASS 6.5.svn (Basilicata):~ >


output
GRASS 6.5.svn (Basilicata):~ > r.ipso.py map=dem@Fiumarella_dataset -ab
image=Fiumarella
100%
Tot. cells 172200.0
===========================
Ipsometric | quantiles
===========================
994 | 0.025
989 | 0.05
980 | 0.1
960 | 0.25
918 | 0.5
870 | 0.75
882 | 0.7
841 | 0.9
817 | 0.975

Done!
GRASS 6.5.svn (Basilicata):~ >


output


import
import sys
import os
import matplotlib.pyplot as plt
import grass.script as grass
import numpy as np

Main (1/4)
def main():
# r.stats gives in the first column the elevation and in the second the number of
cells having that elevation
stats = grass.read_command('r.stats', input = options['map'], fs = 'space', nv =
'*', nsteps = '255', flags = 'inc').split('n')[:-1]

# res = cellsize
res = float(grass.read_command('g.region', rast = options['map'], flags =
'm').strip().split('n')[4].split('=')[1])
zn = np.zeros((len(stats),6),float)
kl = np.zeros((len(stats),2),float)
prc = np.zeros((9,2),float)

for i in range(len(stats)):
if i == 0:
zn[i,0], zn[i, 1] = map(float, stats[i].split(' '))
zn[i,2] = zn[i,1]
else:
zn[i,0], zn[i, 1] = map(float, stats[i].split(' '))
zn[i,2] = zn[i,1] + zn[i-1,2]

totcell = sum(zn[:,1])
print "Tot. cells", totcell


Main (2/4)
for i in range(len(stats)):
zn[i,3] = 1 - (zn[i,2] / sum(zn[:,1]))
zn[i,4] = zn[i,3] * (((res**2)/1000000)*sum(zn[:,1]))
zn[i,5] = ((zn[i,0] - min(zn[:,0])) / (max(zn[:,0]) - min(zn[:,0])) )
kl[i,0] = zn[i,0]
kl[i,1] = 1 - (zn[i,2] / totcell)

# quantiles
prc[0,0] , prc[0,1] = findint(kl,0.025) , 0.025

# Managing flag & plot
if flags['a']:
plotImage(zn[:,3], zn[:,5],options['image']+'_Ipsometric.png','-','A(i) /
A','Z(i) / Zmax','Ipsometric Curve')
if flags['b']:
plotImage(zn[:,4], zn[:,0],options['image']+'_Ipsographic.png','-','A [km^2
]','Z [m.slm]','Ipsographic Curve')


Main (3/4)
print "==========================="
print "Ipsometric | quantiles"
print "==========================="
print '%.0f' %findint(kl,0.025) , "|", 0.025
print 'n'
print 'Done!'
#print prc

def findint(kl,f):
Xf = np.abs(kl-f); Xf = np.where(Xf==Xf.min())
z1 , z2 , f1 , f2 = kl[float(Xf[0])][0] , kl[float(Xf[0]-1)][0] , kl[float(Xf[0])
][1] , kl[float(Xf[0]-1)][1]
z = z1 + ((z2 - z1) / (f2 - f1)) * (f - f1)
return z


Main (4/4)
def plotImage(x,y,image,type,xlabel,ylabel,title):
plt.plot(x, y, type)
plt.ylabel(ylabel)
plt.xlabel(xlabel)
plt.xlim( min(x), max(x) )
plt.ylim( min(y), max(y) )
plt.title(title)
plt.grid(True)
plt.savefig(image)
plt.close('all')

if __name__ == "__main__":
options, flags = grass.parser()
sys.exit(main())

Zn has n rows and 6 columns [len(stats) = n]

Kl has n rows and 2 columns

Prc has 9 rows and 2 columns

Zn

0 1 2 3 4 5

A= B= C = (if i=0, D = (1-C) / E = Di * F = (A –
elevation numbers of Ci=Ai; else numbers of (res^2) / min(A)) /
cells Ci = Ai + B(i- cells 1000000 * (max(A) -
1) ) numbers of min(A))
cells

Zn has n rows and 6 columns [len(stats) = n]

Kl has n rows and 2 columns

Prc has 9 rows and 2 columns

kl
0 1

A= G = 1 – (C /
elevation num of cell)


Prc has 9 rows and 2 columns.
It defines the ipsometric curve by the quantiles of the distribution.
It is built by the function ”findint”
def findint(kl,f):
Xf = np.abs(kl-f); Xf = np.where(Xf==Xf.min())
z1 , z2 , f1 , f2 = kl[float(Xf[0])][0] , kl[float(Xf[0]-1)][0] , kl[float(Xf[0])
][1] , kl[float(Xf[0]-1)][1]
z = z1 + ((z2 - z1) / (f2 - f1)) * (f - f1)
return z

np.abs and np.where are two NumPy routines:

numpy.absolute(x[, out])
Calculate the absolute value element-wise.

numpy.where(condition[, x, y])
Return elements, either from x or y, depending on condition.
If only condition is given, return condition.nonzero().

See NumPy doc at http://docs.scipy.org/doc/


Some useful links:

GRASS and Python on Osgeo wiki:
http://grass.osgeo.org/wiki/GRASS_and_Python
GRASS Python Scripting Library:
http://grass.osgeo.org/programming6/pythonlib.html
Style rules:
http://trac.osgeo.org/grass/browser/grass/trunk/SUBMITTING_PYTHON

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