Overview
This lecture introduced the fundamentals of coordinate systems and map projections, their role in mapping data, and how to resolve issues when data layers do not align in GIS software.
Common Data Alignment Problems
- Data may not display or align correctly if coordinate systems are missing or mismatched.
- Layers with undefined coordinate systems can appear far from their actual location.
Coordinate Systems Basics
- Coordinate systems define location, distance, and direction on the earth.
- Two main types: Geographic Coordinate Systems (GCS, angular units) and Projected Coordinate Systems (PCS, linear units).
- Geographic systems use latitude and longitude; projected systems use x/y or eastings/northings.
- The "WKID" (Well-Known ID) or "WKT" (Well-Known Text) uniquely identifies a coordinate system.
Managing Coordinate Systems in GIS
- Use the Define Projection tool only when data lacks a coordinate system.
- Never overwrite metadata unless certain of the source coordinate system.
- The EPSG registry provides standard codes for coordinate systems and transformations.
Vertical Coordinate Systems
- Vertical coordinate systems define height or depth using a vertical datum and may be based on ellipsoidal or gravity-related (geoid) models.
- EGM2008 is a widely used global geoid model.
Map Projections
- Map projections convert 3D earth surfaces into 2D maps, causing distortions in area, distance, shape, or direction.
- Common projections: Web Mercator (distorts area), Albers Equal Area (preserves area), Stereographic (preserves angles), Equidistant (preserves distance from center).
- No projection preserves all properties; choice depends on map purpose and extent.
Transformations
- Transformations convert data between different coordinate systems or datums.
- Datum transformations can be horizontal or vertical; they are defined for specific areas and have varying accuracy.
- Software typically suggests the best transformation based on data and coordinate system extents.
Practical Steps in Software
- Always check data’s coordinate system properties before working.
- Use spatial filters or search by WKID to identify or assign proper systems.
- Custom coordinate systems can be created if not available by default.
- When mixing coordinate systems, specify the appropriate transformation to ensure accurate alignment.
Key Terms & Definitions
- Coordinate System — Framework for defining locations in space (geographic or projected).
- WKID/WKT — Unique identifiers/text for coordinate system specifications.
- Projection — Mathematical transformation from 3D to 2D.
- Datum — Reference model for measuring locations on Earth.
- Transformation — Mathematical process to convert between datums or vertical references.
- Geoid — Gravity-related, irregular reference surface for measuring elevation.
Action Items / Next Steps
- Review the assigned PDF on geographic transformations and accuracy.
- Check for and install the latest ArcGIS coordinate system data setup if needed.
- Consult the EPSG online registry and GIS help documentation for details on codes and transformations.
- Practice assigning and verifying coordinate systems in your GIS projects.