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

Questions?

Contact:

Jesse Wickizer

jesse@maps.com