Projections and Coordinate Reference Systems

Visualizing Geospatial Data in Python

Mary van Valkenburg

Data Science Program Manager, Nashville Software School

Projections

flattened map of spherical earth

Many approaches to map projection

xkcd comic on map projections

Coordinate Reference Systems

EPSG:4326

Used by Google Earth
Units: decimal degrees

EPSG:3857

Used by Google Maps, Bing Maps, Open Street Maps
Units: meters

School Name              Latitude     Longitude
A. Z. Kelley Elementary    36.021       -86.658
Alex Green Elementary      36.252       -86.832
Amqui Elementary           36.273       -86.703
Andrew Jackson Elementary  36.231       -86.623

import geopandas as gpd
schools['geometry'] = gpd.points_from_xy(schools.Longitude, schools.Latitude)
schools.head(4)

School Name                Latitude  Longitude                geometry
A. Z. Kelley Elementary      36.021    -86.658  POINT (-86.658 36.021)
Alex Green Elementary        36.252    -86.832  POINT (-86.832 36.252)
Amqui Elementary             36.273    -86.703  POINT (-86.703 36.273)
Andrew Jackson Elementary    36.231    -86.623  POINT (-86.623 36.231)

Creating a GeoDataFrame from a DataFrame

import geopandas as gpd

schools_geo = gpd.GeoDataFrame(schools, 
                               crs = 'epsg:4326', 
                               geometry = schools.geometry)

schools_geo.head(4)

School Name                Latitude  Longitude               geometry
A. Z. Kelley Elementary      36.021    -86.658  POINT (-86.658 36.021)
Alex Green Elementary        36.252    -86.832  POINT (-86.832 36.252)
Amqui Elementary             36.273    -86.703  POINT (-86.703 36.273)
Andrew Jackson Elementary    36.231    -86.623  POINT (-86.623 36.231)

Changing from one CRS to another

schools_geo.head(2)

School Name              Latitude  Longitude                geometry
A. Z. Kelley Elementary    36.021    -86.658  POINT (-86.658 36.021)
Alex Green Elementary      36.252    -86.832  POINT (-86.832 36.252)

schools_geo.geometry = schools_geo.geometry.to_crs(epsg = 3857)
schools_geo.head(2)

School Name              Latitude  Longitude                      geometry
A. Z. Kelley Elementary    36.021    -86.658  POINT (-9646818.8 4303623.8)
Alex Green Elementary      36.252    -86.832  POINT (-9666119.5 4335484.4)

Let's practice!

Visualizing Geospatial Data in Python