By the end of this lesson, you should be able to:
- Recognize when specific projections should be used for GIS tasks.
- Illustrate the use of coordinate system transformations.
- Explain how coordinate system and projection differences at different scales may affect measurements and GIS operations.
- Determine appropriate projections for scale and extent of map.
B. Project Overview
Geospatial data is referenced to the surface of Earth by having locational coordinates either define the vertices of point, line and polygon (i.e. vector) features, or be associated with the cells of raster datasets. These coordinates are either spherical or planar. Longitude and latitude, represented by angular degree values, are the spherical coordinates we use. When the surface of Earth is flattened in the process of being mapped, those spherical coordinates are converted to planar (or 2-dimensional Cartesian) coordinates, most often given in units of meters or feet.
You were introduced to map projections and datums in Geography 482. In Geography 483 you performed minimal projection change and projection definition operations, in the context of shapefile datasets. In Geography 484 you were hit a bit harder with the differences between reprojecting a dataset (thereby creating a new dataset) and changing the definition of the coordinate system of a dataset. In Geography 484 it was all in the context of the personal geodatabase.
Learn more about datums and projections in the Concept Gallery.
Understanding coordinate systems and knowing how to deal with them in the GIS software environment are things that we believe you can not get enough exposure to. In this exercise we will get you to think a bit more about the appropriateness of a given projection, depending upon the nature of the information being mapped. You will also get practice with more of the tools that give you control over supplying coordinate system definitions to datasets and with converting to different coordinate systems. Most significantly, you will learn more of the specifics of datum-level coordinate transformation.
You will work with both coverages and shapefiles, ultimately converting those datasets into personal geodatabase feature classes. (Recall that the coverage, shapefile and feature class are all formats in which vector data are stored by the Esri software.) The layers we deal with in this lesson are almost all comprised of polygon features. A polygon feature class attribute table, by default, includes a field that contains the area of each polygon feature. Shapefile attribute tables do not, by default, have an area field. Coverage attribute tables for polygon coverages do automatically create an area field, but coverages cannot be created in ArcView.
In parts of the lesson there are step-by-step instructions for much of what you will do. There are also places where you are expected to be able to perform certain tasks without stepwise instructions. These are tasks that either you have just been taught, were shown to you in previous courses, or, you should be able to figure out if you were qualified to skip previous courses. (Don't worry, there are usually hints as to what to do, and you can always ask for help.)
Upon completion of this lesson, you will have gained a better understanding of the implications of the coordinate system in which geographic data is cast and how to control it. You should gain confidence in knowing when adequate coordinate system information is associated with a dataset. And you should be more comfortable with the GIS tools that allow you to control the coordinate system parameters. Only then can you be confident that datasets cast in differing coordinate systems will overlay properly.
You will also increase your understanding of map projections in the context of properly presenting spatial information.
- Understanding the subtleties of map projections, from the standpoint of the GIS display and analysis tools
- Dealing with datum transformations
- Gaining a healthy appreciation of the capabilities and the limitations of the GIS software
- Learning more about map scale and its implications when it comes to the measurement of area
- More experience with WorldWideWeb-available data
C. Lesson Checklist
Lesson 7 is one week in length. (See the Calendar in Canvas for specific due dates.) To finish this lesson, you must complete the activities listed below. You may find it useful to print this page out first so that you can follow along with the directions.
|1||Read the concepts introduced in the lesson.||See the Concept Gallery page to begin.|
Work through the Lesson 7 Project.
|See the Part I: Getting Started page to begin.|
|3||Submit the Lesson 7 Project.||Detailed instructions located on the Lesson Project Tasks page.|
|4||Submit two peer reviews of proposals.||
See Writing the Project Proposal for directions for submitting two peer reviews of Proposals. Note: this is due the Saturday in the middle of Lesson 7.
Complete the Lesson 7 Quiz.
|Complete the Lesson 7 Quiz in Canvas.|
If you have any questions now or at any point during this week, please feel free to post them to the Lesson 7 Discussion Forum. While you are there, feel free to post your own responses if you, too, are able to help a classmate.