Lecture at North Carolina State University about python programming in GRASS GIS environment

1. Python scripting in GRASS GIS environment
Geospatial Analysis and Modeling – MEA792
Margherita Di Leo

2. Python scripting in GRASS GIS environment
Python + GRASS GIS:
➢ Python scripts that call GRASS functionality from
outside
➢ Running external commands from Python (as a
grass module)

3. Python scripting in GRASS GIS environment
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/

4. Python scripting in GRASS GIS environment
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

5. Python scripting in GRASS GIS environment
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

6. Python scripting in GRASS GIS environment
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):~ >

7. Python scripting in GRASS GIS environment
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):~ >

8. Python scripting in GRASS GIS environment
output

9. Python scripting in GRASS GIS environment
output

10. Python scripting in GRASS GIS environment
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

11. Python scripting in GRASS GIS environment
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
prc[1,0] , prc[1,1] = findint(kl,0.05) , 0.05
prc[2,0] , prc[2,1] = findint(kl,0.1) , 0.1
prc[3,0] , prc[3,1] = findint(kl,0.25) , 0.25
prc[4,0] , prc[4,1] = findint(kl,0.5) , 0.5
prc[5,0] , prc[5,1] = findint(kl,0.75) , 0.75
prc[6,0] , prc[6,1] = findint(kl,0.9) , 0.9
prc[7,0] , prc[7,1] = findint(kl,0.95) , 0.95
prc[8,0] , prc[8,1] = findint(kl,0.975) , 0.975
# 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')

12. Python scripting in GRASS GIS environment
Main (3/4)
print "==========================="
print "Ipsometric | quantiles"
print "==========================="
print '%.0f' %findint(kl,0.025) , "|", 0.025
print '%.0f' %findint(kl,0.05) , "|", 0.05
print '%.0f' %findint(kl,0.1) , "|", 0.1
print '%.0f' %findint(kl,0.25) , "|", 0.25
print '%.0f' %findint(kl,0.5) , "|", 0.5
print '%.0f' %findint(kl,0.75) , "|", 0.75
print '%.0f' %findint(kl,0.7) , "|", 0.7
print '%.0f' %findint(kl,0.9) , "|", 0.9
print '%.0f' %findint(kl,0.975) , "|", 0.975
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

13. Python scripting in GRASS GIS environment
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())

14. Python scripting in GRASS GIS environment
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

15. Python scripting in GRASS GIS environment
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)

16. Python scripting in GRASS GIS environment
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/

17. Python scripting in GRASS GIS environment
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