Tags: gisc 4500k, lectures, presentations, python

Accessing raster data using ArcPy

GISC 4500K

Dr. Huidae Cho

Institute for Environmental and Spatial Analysis...University of North Georgia

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Contents

1 ArcPy modules
2 ArcGIS Pro Python window interface
3 Help
4 Project
5 Maps
6 Layers
7 Raster data
- 7.1 Add raster layers
- 7.2 Raster
- 7.3 Raster attributes
- 7.4 Raster to array
- 7.5 Raster bands
- 7.6 Modifying cell values
- 7.7 Save raster

1 ArcPy modules

arcpy.mp: Mapping module
arcpy.sa: Spatial analyst module
arcpy.da: Data access module
arcpy.na: Network analyst module
arcpy.sharing: Sharing module
arcpy.wmx: Workflow manager module

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2 ArcGIS Pro Python window interface

(P)	Packages
(O)	Objects
<M>	Methods
<F>	Functions
<T>	Tools
( )	Constants
(...)	Groups

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3 Help

help(arcpy)
help(arcpy.mp)
help(arcpy.mp.ArcGISProject)

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4 Project

In most cases, we start from an ArcGISProject object (a project).

# specific project
prj = arcpy.mp.ArcGISProject(r'P:\YOUR_PATH\Project.aprx')

# current project
prj = arcpy.mp.ArcGISProject('CURRENT')

# help on an object?
help(prj)

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5 Maps

An ArcGISProject may contain multiple Maps.

# all maps
for m in prj.listMaps():
    print(m.name)

# active map
m = prj.activeMap
print(m.name)

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6 Layers

Usually, there are multiple Layers in a Map.

# all layers in the active map
for lyr in m.listLayers():
    print((lyr.name, lyr.dataSource, lyr.isRasterLayer, lyr.isFeatureLayer, lyr.isWebLayer))

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7 Raster data

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7.1 Add raster layers

Add your own raster layers or create a new one.

# let's export the current map as a color TIFF
filename = r'P:\YOUR_PATH\map.tif'
width = height = 1024
m.defaultView.exportToTIFF(filename, width, height, geoTIFF_tags=True)

# add the TIFF to the map
m.addDataFromPath(filename)

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7.2 Raster

This is how you find a raster Layer and get its Raster object.

# find the first raster layer
for lyr in m.listLayers():
    if lyr.isRasterLayer:
        break

# get the raster object
ras = arcpy.Raster(lyr.name)
print(ras.name, ras.extent)

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7.3 Raster attributes

A Raster object has many useful attributes.

# all attributes not starting with _
for i in dir(ras):
    if not i.startswith('_'):
        # ras.attribute_name is equivalent to getattr(ras, attribute_name)
        # why use this syntax? when attribute_name is string
        # e.g., ras.extent == getattr(ras, 'extent')
        print((i, getattr(ras, i)))

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7.4 Raster to array

The Raster object does not expose its cell values as an attribute.

Then, how can you access/manipulate the cell values?

Convert the Raster object to a NumPy array. NumPy arrays are a GIS-agnostic data type. We can use NumPy functions to modify raster cells.

arr = arcpy.RasterToNumPyArray(ras)
print(arr)

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7.5 Raster bands

Some raster Layers have four bands including red, green, blue, and alpha.

print(arr.shape)

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7.6 Modifying cell values

Just use the NumPy array to change cell values.

# clone arr
arr2 = arr.copy()

# set the entire red band to 0
arr2[0] = 0

# set the green band at cell (50, 60) to 255
arr2[1][50][60] = 255

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7.7 Save raster

Save the NumPy array as a raster file.

filename2 = r'P:\YOUR_PATH\map_wo_red.tif'
ras2 = arcpy.NumPyArrayToRaster(arr2, ras.extent.lowerLeft, ras.meanCellWidth, ras.meanCellHeight)
arcpy.CopyRaster_management(ras2, filename2)
m.addDataFromPath(filename2)

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