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Lab Assignment 04 - Map Projections
Introduction
This lab focuses on a one fundamental function of a Geographic Information System – Its ability to translate spatial coordinates stored either as longitudes/latitudes or in a specific map projection into another. As with other modern GIS systems, ArcGIS presents the user to accomplish this in two ways. ArcMap has the ability to project geographic data stored as either rasters or vectors from the geographic coordinate system it is stored in to the one specified for display. This is accomplished in real time and is often informally referred to as projecting “on the fly.” GIS systems (in the case of ArcGIS the functionality is found within ArcToolbox ) also provide a user the ability to permanently transform a geographic coverage from one map projection into another. Both of these approaches have their advantages and disadvantages.
Before you begin learning how to implement map projections in a GIS, a process which has gotten much easier and much better over time, a few pertinent points need to be made.
First and foremost, while a GIS can perform transform geographic coordinates from any number of map projections to another, it is up to the GIS user to understand the strengths/weaknesses of a particular map projection and how appropriate its intended use.
Within a GIS spatial coordinates can be stored either in spherical coordinates (latitude/longitudes) or in Cartesian coordinates (x,y within a particular map projection). GIS coverages where the spatial information is stored as latitude and longitudes is often said to be unprojected and can be more
formally said to be in ageographic coordinate system (GCS ). Conversely, a GIS coverage whose
spatial coordinates are stored as Cartesian coordinates within a specified map projection are often
said to be projected and are formally said to be in a projected coordinate system (PCS).
Because the mathematics of dealing with information in spherical and Cartesian coordinates differ greatly, for most GIS analytical operations it is required that the data be projected – or stored in a projected coordinate system. However, if the primary use of a particular GIS theme is primary for visualization e.g., making a world map, it may be preferable to store the coordinate information in a geographic coordinate system.
Proper use of datums is has become very important in GIS. Whereas in the past, many GIS systems accommodated datum differences poorly, if at all, today’s GIS systems properly handle the different ellipsoids, tie points, etc. used by different datums. Proper use of datums is most important for large-scale mapping, where a location difference of 200 m attributable to incorrectly specifying a datum may be inconsequential for an 8.5 by 11 world map, it may be huge when using a GIS to locate a water well in a farm field.
GIS offers flexibility in creating maps with map projections tailored specifically to the problem at hand unheard in old-fashioned pen and ink cartography. Take advantage of this technology, but be sure to use it correctly.
Part 1 – Mastering ArcGIS™
This week you work on a very important chapter in Mastering ArcGIS – Chapter 11 Coordinate
Systems. You need to take time to read the Mastering the Concepts section before beginning with the computer. You need to make sure you especially know the difference between the Project Define Wizard and the Projection Wizards. If you incorrectly apply one of these tools, your lab will become 10 times harder.
This tutorial also introduces a new part of ArcToolbox. As the name implies ArcToolbox is the part of ArcMap that provides GIS analytical capabilities, data ingestion routines and the ability to work with map projections in ways not available in ArcMap. As it currently is designed, it is a largely wizard-driven interface to undertake GIS tasks. It is possible to do some batch processing (e.g. convert multiple GIS coverages from one projection to another at the same time). Because your book guides you through one example of using ArcToolbox, (defining projections) I will let you figure out the steps required in using the others as the approach is similar, it is just the dialog boxes that change.
I do want to introduce you to a couple of ArcGIS specific issues when dealing with map projections and point out a few things which might not be emphasized enough in the tutorial.
- Get in the habit of defining the map projection of your layers when you begin (coordinate system is one of the layer properties). Once you do this, all data you add (where ArcMap sufficiently understands what projection the data is stored in) will be projected on the fly into your specified projection. If you do not specify the projection, then ArcMap will default to the projection of the first layer (coverage) you add.
- Get used to using ArcGIS help or other documentation to help you select the correct map projection for your work. When I was a graduate student our labs copy of An Album of Map Projections was dog-eared because of its use as a reference.
- One thing that IMHO (in my humble opinion) is not emphasized enough in the tutorial is that when ArcMap projects data “on the fly” it uses approximations to the exact mathematical transformations required to go between different map projections. It does this for speed, but if you need to do more exacting analysis you need to do a permanent transformation. I say this because it is possible to quite easily export a theme as a new dataset from ArcMap – just select a layer and right-click and select Export Data. It does allow you the option to save in the coordinate system, but this apparently does not do an exacting reprojection (converting from one map projection to another).
- One of the most common mistakes when using ArcToolbox to project data is a
misunderstanding between defining a map projection and actually projecting it. This often
stems from inadequate understanding of map projections.
The single spatial locations (x,y) that represent a point or the suite of x,y locations that define a line or a polygon are stored as Cartesian coordinates in a projected coordinate system. For example a point may have an x,y of 734,342, 1,432,031 meters. This coordinate pair fully defines its location in that coordinate system as well as correctly gives its position relative to other
features in that coordinate system. However, for GIS to be able to compare this location to
other locations on the spherical earth or to coordinates in other coordinate systems, it must have
Well, that is probably enough additional information. Take your time and complete the tutorial then on to the graded assignment…
Part 2 – Graded Assignments
First, you should complete Exercises 2, 3, and 4 in the Mastering ArcGIS manual. Note: GEOG 660 students do not have to complete these exercises
In this lab you will be graded on (1) your ability to demonstrate technical competency in using GIS to perform various map projection techniques, (2) your understanding of map projections and ability to select an appropriate map projection given the purpose of the required analysis and/or its visual presentation, and (3) appropriate high quality cartography. I am going to continue to strive to push good map making skills here and while I am not going to require you spend hours perfecting your maps, I want you to spend a little some making sure your map is cartographically suitable. Often a little effort can result in a much better looking map. Remember, how confident people will feel about your work is often influenced by how well it is presented.
You will be expected to demonstrate proficiency in altering the projections for display in ArcMap and reprojecting data using ArcToolbox. In this sense some of the requirements for what you turn in may differ from what would be expected in the “real-world.” Enjoy the assignment, but take time to do it well – a proper understanding of map projections is important and is a GIS skill you will use repeatedly.
Also, please turn in both hardcopies of your maps and associated memo and the ArcMap projects you used to create them to your TAs.
GOOD LUCK AND HAVE FUN!!
”Altering people’s view of the world for over 50 years”
September 28, 2009
TO: GIS Intern 1st^ Class
FROM: Dr. Andrew G. Klein, President and CEO
RE: Nunavut Mapping
I have heard from your supervisor that you have done a very good job with the Texas Transportation Corridor Map. Now we need you to go international! We are trying to build close connections with our neighbor in the Great White North. Specifically, we are trying to aid the newly formed Nunavut Territory in northern Canada with GIS. Nunavut is the largest, and newest, of Canada’s territories formed from part of the Northwest Territories on April 1 st, 1999. For more information on Nunavut I would encourage you to visit the site: http://www.gov.nu.ca
In order for our company to begin working on this project we need to generate some basic maps so our employees know where Nunavut is in relationship to the other Canadian territories and provinces. Specifically, I would like you to do the following:
Permanently reproject basic vector and raster GIS themes into a conformal map projection suitable for the entire country of Canada. Your supervisor will inform you where the necessary GIS themes reside. Once you have projected the files, using ArcCatalog you should print out both a preview and the spatial attribute metadata for each reprojected GIS theme to show your supervisor you have done the projection correctly.
Using the reprojected GIS themes, you should create a conformal map of Canada that highlights Nunavut and its capital Iqaluit relative to the other Canadian providences territories and capitals as well as Canada’s major infrastructure. Like your Texas map, you might wish to show Canada’s physiography as a base layer.
Finally, to highlight the size of this new territory, you should create a second map (or an inset map for your first map) that correctly compares the areas of Nunavut and our own state, Texas. Both of these states should be highlighted relative to the other states, provinces and territories. You do not have to reproject any GIS themes to create this map, you can simply use ArcMap’s ability to project data on the fly. Add the projection information to the map so we know what projection you used.
