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Civil Engineering Transportation GIS Workshop

workshop guide

Coordinate Systems

A reference system used to represent and integrate data layers. Two common types of coordinate systems used:
  • Geographic coordinate system: locations defined on earth using a three-dimensional spherical surface (sphere or spheroid)
    • A point is referenced by its longitude and latitude values.
  • Projected coordinate system: defined on a flat, two-dimensional surface.
    • Has constant lengths, angles, and areas across the two dimensions.
    • Always based on a geographic coordinate system.

Source & Additional Information - ESRI Guide: What are map projections?

Esri Training: Basics of Map Projections

Geographic Transformations

  • The same location on the ground will have different coordinate values in different geographic coordinate systems.
  • Displaying data in a projected coordinate system can exaggerate the differences which can be from a few centimeters to hundreds of meters.
  • If you are using datasets that are based on different geographic coordinate systems, you will need to set a geographic transformation.

Source & Additional Information - ESRI Guide: Choosing an appropriate transformation

Issue: Datasets do not line up when added to ArcMap.

When adding data to ArcMap, the application reads the coordinate system (projection) information stored with the dataset. When displaying multiple datasets with different coordinate systems, ArcMap will project the data 'on-the-fly' to the coordinate system that is set for the Data Frame.
  • If there is no coordinate system associated with the data, or
  • If the coordinate system definition is incorrect, or
  • If the proper geographic (datum) transformation is not specified,

the data will not line up properly with other data displayed in the map document. 

Additional Information & Source - Technical Article: Problem: Datasets do not line up when added to ArcMap

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