Working with Geospatial Data: Advanced

Working with Geospatial Data: Advanced#

Place matters. After covering the intermediate tutorial, you’re ready to cover some advanced spatial analysis topics.

Below are more detailed explanations for dealing with geometry in Python.

Getting Started#

# Import Python packages
import pandas as pd
import geopandas as gpd
from shapely.geometry import Point
from geoalchemy2 import WKTElement

Types of Geometric Shapes#

There are six possible geometric shapes that are represented in geospatial data. More description here.

  • Point

  • MultiPoint: collection of points

  • LineString

  • MultiLineString: collection of linestrings, which are disconnected from each other

  • Polygon

  • MultiPolygon: collection of polygons, which can be disconnected or overlapping from each other

The ArcGIS equivalent of these are just points, lines, and polygons.

Geometry In-Memory and in Databases#

If you’re loading a GeoDataFrame (gdf), having the geometry column is necessary to do spatial operations in your Python session. The geometry column is composed of Shapely objects, such as Point or MultiPoint, LineString or MultiLineString, and Polygon or MultiPolygon.

Databases often store geospatial information as well-known text (WKT) or its binary equivalent, well-known binary (WKB). These are well-specified interchange formats for the importing and exporting of geospatial data. Often, querying a database (PostGIS, SpatiaLite, etc) or writing data to the database requires converting the geometry column to/from WKT/WKB.

The spatial referencing system identifier (SRID) is the geographic coordinate system of the latitude and longitude coordinates. As you are writing the coordinates into WKT/WKB, don’t forget to set the SRID. WGS84 is a commonly used geographic coordinate system; it provides latitude and longitude in decimal degrees. The SRID for WGS84 is 4326. Refresher on geographic coordinated system vs projected coordinated system.

Shapely is the Python package used to create the geometry column when you’re working with the gdf in-memory. Geoalchemy is the Python package used to write the geometry column into geospatial databases. Unless you’re writing the geospatial data into a database, you’re most likely sticking with shapely rather than geoalchemy.

To summarize:

Data is used / sourced from…

Python Package

Geometry column

SRID/EPSG

Local Python session, in-memory

shapely

shapely object: Point, LineString, Polygon and Multi equivalents

CRS is usually set, but most likely will still need to re-project your CRS using EPSG

Database (PostGIS, SpatiaLite, etc)

geoalchemy

WKT or WKB

define the SRID

 
# Set the SRID
srid = 4326
df = df.dropna(subset=['lat', 'lon'])
df['geometry'] = df.apply(
    lambda x: WKTElement(Point(x.lon, x.lat).wkt, srid=srid), axis = 1)

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