McConkey: Chapter 9

This chapter was fairly easy to read and was great for starting to understand how geometry objects (something everyone who has worked with ArcGIS should be familiar with) works with Python.

Chapter 9 goes over how to work with geometries, otherwise known as points, polylines, and polygons. This includes being able to read the properties of geometry objects from preexisting features as well as creating geometry objects with coordinate information.

The chapter explains how to use geometry cursors which allow you to work with or edit the vertices of a geometry feature object. To set a cursor you must use a geometry token rather than a field name.  For example, the SHAPE@ token gives access to the full geometry object and its properties.  Using specific geometry tokens may be easier such as SHAPE@XY, or SHAPE@LENGTH.  (SHAPE@XY results in a tuple of x,y coordinates for the feature’s centroid, and the SHAPE@LENGTH returns the feature’s length.)

When working with a polygon with holes, you may have some trouble. A geometry object that consists of a polygon with holes will return an exterior ring with one or more interior rings. Rings are made up of paths, or a series of vertices that begin with a starting vertex and finish with an end vertex. Exterior rings are clockwise and interior rings are counterclockwise. (For more info on polygons with holes look on page 299.)

There are several classes that can be used to make geometry objects. By using the Point object you can create a single vertice. You can also make multiple  Point objects to create polylines or polygon features.