Datums, Projections, and Spatial Reference Systems
Table of Contents
- Geospatial Analysis Skills: Review and Self-Assessment
- Setting up Your Workspace
- Datums, Projections, and Spatial Reference Systems
- Working with Database Tables
- Adding XY Data
- Proximity and Overlay Operations
Previously, you learned that geospatial scientists model the surface of Earth by transforming the irregular non-geometrical shape of the geoid surface model into a smooth mathematically defined surface model called the reference ellipsoid. You also learned that because the geoid and the reference ellipsoid are not an exact match, there are multiple reference ellipsoids used today, each optimized for different regions of Earth.
Additionally, you also were taught that when you combine a reference ellipsoid with a network of carefully surveyed control points, establish a geodetic datum. Geodetic datums are standard reference points or reference ellipsoids that serve as a base for calculating positions on and above the surface of Earth. You also learned that geodetic datums provide a starting point for creating a map projection. A map projection is a geometric transformation of the round Earth onto a flat plane using mathematical equations. The process of transforming a spherical object onto a flat plane distorts area, shape, distance, direction, and continuity. These are known as the five preserved properties because some map projections can maintain one or more of these characteristics with a high degree of accuracy. However, no map projection can retain all five of these qualities at the same time.
Finally, you learned about different types of spatial reference systems, including geographic coordinate systems and projected coordinate systems. A geographic coordinate system (GCS) is a coordinate-based spatial reference system using latitude and longitude. An important fact to remember is that a geographic coordinate system is ellipsoid-based, which makes it useful for defining a location on the surface of the earth. However, a geographic coordinate system makes measuring distances and areas complicated. A projected coordinate system solves this problem by using map projections to transform a spherical system, like latitude and longitude, into a flat two-dimensional grid. Projected coordinate systems use feet or meters to measure distances and areas, making them ideal for calculations. Such systems include the Universal Transverse Mercator (UTM) system and the State Plane Coordinate (SPC) system.
In the following skill drills, you will review how to check the datums, projections, and spatial reference systems using ArcGIS software.
Skill Drill: Download and Decompress Data
Using the following link, download the 7zip file provided. Save the file to your original folder:
In this tutorial, you are required to use 7zip, a file decompression software that handles multiple file compression formats commonly used for geospatial data. If you are working from home, you may need to install 7zip on your personal computer. Watch Installing 7zip on Windows 10, produced by Nicolas R. Malloy, to learn more (Figure 1.3).
Skill Drill: Repairing Broken Links
Once you have decompressed the contents of the 7z file, locate the map document file (.mxd) named Projections Datums and Spatial Reference Systems.mxd (Figure 1.4). Double-click to open the file.
The data frame appears empty, and you should see red exclamation points preceding each layer in the Table of Contents (Figure 1.5). Using the procedures you learned in previous tutorials, repair the broken links to the data sources.
Skill Drill: Verify the spatial reference information
After repairing the broken links, you should see three datasets displayed on the data frame. Previously, you learned that in ArcGIS, the data frame window shows the map using project-on-the-fly. Even though the layers in the Table of Contents might use different projections and coordinate systems internally, ArcGIS tries to line them up on the screen using the coordinate system defined in the data frame properties. What this means is that you can change the projection in the data frame to alter the appearance of the map onscreen without modifying the original data files.
Using the skills you learned earlier, open a blank Microsoft Word document, and record the answers to the following questions as they apply to the current map document and data frame.
1. What datum does the current data frame use to project on the fly?
2. What map projection does the current data frame use to project on the fly?
3. What geographic coordinate system (GCS) does this data frame use?
4. What projected coordinate system (PCS) does this data frame use?
5. Which layer in the table of contents uses the same projected coordinate system as this data frame?
6. Which layer in the table of contents has a projected coordinate system that is different from this data frame?
7. Which layer in the table of contents does not have a projected coordinate system?
8. If you wanted to create a California layer that used different spatial reference system, datum, and projection, which tool in ArcMap would you need to use?
9. If you wanted to create a CA hypsometric tiff file that used different spatial reference system, datum, and projection, which tool in ArcMap would you need to use?
Hint: For questions 1-9, review the skills you learned in the Chapter 3 Tutorial Working with Projections, from the Geospatial Concepts Text. Also, you learned about geographic coordinate systems and the different projected coordinate systems in Chapter 4.
Skill Drill: Inspect the Metadata
Using the skills you learned earlier, make sure that the metadata style in the ArcMap options is set to the ISO19139 Metadata Implementation Specification. For the next few questions, open the metadata for each file. Search through the metadata to find the answers.
Hint: If you do not remember how to change the metadata settings and open the metadata, review the skills you learned in the Chapter 1 Tutorial Basic Quality Assurance and Quality Control.
In your Microsoft Word document, record the answers to the following questions using the information you find in the metadata.
10. What was the last geoprocessing tool used on the file CA_hypsometric.tif?
11. In the California counties shapefile, what does the attribute COUNTYFP stand for?
12. Who is the source for the description of the COUNTYFP attribute?
13. In the United States shapefile, what is the geographic extent (north, south, east, west) in decimal degrees latitude and longitude?
Hint: For questions 10-13, review the skills you learned in the Chapter 1 Tutorial Basic Quality Assurance and Quality Control, from the Geospatial Concepts Text.