Database Reference
In-Depth Information
Chapter 3. Working with Vector Data -
The Basics
In this chapter, we will cover the following recipes:
• Working with GPS data
• Fixing invalid geometries
• GIS analysis with spatial joins
• Simplifying geometries
• Measuring distances
• Merging polygons using a common attribute
• Computing intersections
• Clipping geometries to deploy data
• Simplifying geometries with PostGIS topology
Introduction
Inthischapter,youwillworkwithasetofPostGISfunctionsandvectordatasets.You
willfirsttakealookathowtousePostGISwith
GPS data
—youwillimportsuchdata-
sets using
ogr2ogr
, and then compose polylines from point geometries using the
ST_MakeLine
function.
Then, you will see how PostGIS manages and helps you find and fix invalid geo-
metrieswithfunctionssuchas
ST_MakeValid
,
ST_IsValid
,
ST_IsValidReason
,
and
ST_IsValidDetails
.
We will then learn about one of the most powerful elements of a spatial data-
base—spatial joins. PostGIS provides you with a rich set of operators, such as
ST_Intersects
,
ST_Contains
,
ST_Covers
,
ST_Crosses
,and
ST_DWithin
,for
this purpose.
After that, you will use the
ST_Simplify
and
ST_SimplifyPreverveTopology
functions to simplify (generalize) geometries when you don't need too many details.
Whilethisfunctionworkswellonlineargeometries,topologicalanomaliesmaybein-
troduced for polygonal ones. In such cases, you should consider using an external
GIS tool such as
GRASS
.
