Your projection choice means more than just map display
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Transcript of Your projection choice means more than just map display
How Coordinate System and Projection choice affect ArcGIS Geoprocessing Results
WLIA Conference, 2011
Your projection choice means more than just map display
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2 Case Studies
1. Large Scale map – parcel selection by buffer
2. Small Scale map – Geodesic vs Euclidian buffers
Parcel Selection by Buffer
Need list of parcels 1,000 feet from an address
Using ArcMap for the task Using World Street Map ArcGIS Map
Service as base map Parcel data from Milwaukee County
Add World Street Map ArcGIS Map service
Geocode focus address – 3131 S 29th St, Milwaukee
Add address point to map
Convert graphic to a shapefile
Create 1,000 ft bufferfrom address point -default settings
Create 1,000 ft buffer from address point - local coordinate system
Comparison of different buffer outputs
Select parcels that intersect 1st buffer (Web Mercator) – 126 parcels
Select parcels that intersect 2nd buffer (SPCS WI South) – 239 parcels
Geodesic vs Euclidian distance
Geodesic Distance – distance measured along Earth’s surfacea.k.a. Great circle distance
Euclidian Distance – straight line distance between 2 points on a 2D planeMeasured on the Cartesian coordinate
grid of the map projection
Geodesic vs Euclidian distance
ArcGIS can create buffers using either distance
The difference more noticeable at smaller scales and at areas of the map with more distortion
Example - World mapEuclidian distance buffers created
using Equidistant Cylindrical projection