In the final
module of this course, Module 6, we will explore the topic of geometries. This
includes learning how to interpret the properties of geometry objects derived
from existing features, as well as the process of creating new geometry objects
based on coordinate information.
Features are
objects that are depicted on a map or within a dataset. A point represents a
single location like a fire hydrant or a tree; a polyline represents a linear
feature such as a road, river, and a polygon feature that represents an area or
region, like a lake, park, or administrative boundary. Each feature in a data
set is typically represented by a row in an attribute table.
A point feature
class in GIS represents a single location consists of a single vertex defined
by x,y coordinates. On the other hand, polyline and polygons features consist
of multiple vertices and are constructed using two or more points objects.
These vertices define the shape of a polyline or polygon feature.
To effectively
work with geometry objects, it is essential to establish a cursor on the
geometry field. Tokens such as SHAPE@ provide access to the complete geometry
object; however, in the context of a large dataset, this approach may lead to
slower performance. For scenarios where only specific properties are required,
alternative tokens can be utilized. For instance, SHAPE@XY returns a tuple of x
and y coordinates that represent the centroid of the feature, while
SHAPE@LENGTH provides the length of the feature.
ArcPy provides
a variety of classes designed for working with geometry objects, including the
generic arcpy.Geometry class, which is utilized to create geometry objects. In
addition to this, ArcPy includes four specific geometry classes: MultiPoint,
PointGeometry, Polygon, and Polyline. Furthermore, ArcPy incorporates two
additional classes that facilitate the construction of geometry: Array and
Point. The relevant classes include arcpy.Array() and arcpy.Point().
Certain
features are composed of multiple parts but are represented as a single feature
within the attribute table; these are referred to as multipart features. To
determine whether a feature is classified as a single part or multipart, the
isMultipart statement is utilized. Additionally, the partCount function will
provide the total number of geometry parts associated with a given feature.
Furthermore, polygons that contain holes present a challenge. These shapes
typically feature one exterior ring, which is defined as a clockwise ring, and
one or more interior rings that are defined as a counterclockwise ring.
Lastly, we
explored the process of writing geometries, which encompasses the creation of
new features. New features can be developed utilizing the InsertCursor class
from the arcpy.da module. This process requires creating a geometry object, and
then saving the result as a feature using insertRow() method.
cursor =
arcpy.da.InsertCursor()
The lab
assignment for Module 6 requires the development of a Python script that
generates a new text file. This script should populate the file with the Object
Identifier (OID) of each feature or row, the vertex ID, the X and Y coordinates
of the vertices, as well as the name of the river feature derived from a
feature class containing polylines representing rivers in Maui, Hawaii.
Initially, I
imported all necessary modules and classes, enabled the overwriteOutput
setting, configured the workspace, and defined the feature class variable.
Subsequently, I created a rivers.txt file in write mode ("w") and
established a search cursor for the rivers.shp file. This cursor was designed
to access the OID, SHAPE, and NAME fields.
Furthermore, I
implemented a for loop to iterate through each row or feature within the
cursor/ShapeFile, while also creating a variable to serve as a vertex ID
number. A second for loop was introduced to iterate through each point or
vertex in the row, utilizing the .getPart() method to extract the x and y
coordinates of the vertices, incrementing the vertex ID number with each
iteration.
The
output.write() method was employed to append a line to the .txt file, detailing
the Feature/ROW OID, Vertex ID, X coordinate, Y coordinate, and the name of the
river feature. A print statement was also included to display the output.write
result. Finally, I ensured that the .txt file was properly closed and deleting row and cursor variables outside of all loops.
The execution
of the script for Module 6 generated a formatted text file corresponding to
each river feature within the Maui rivers feature class.
Module 6 script flowchart
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