Obtaining the Tutorial Data

Before beginning the tutorial, you will copy all of the required tutorial data onto your Desktop. Follow the applicable set of instructions below depending on the particular computer you are using.

OPTION 1: Accessing tutorial data from Fondren Library using the gistrain profile

If you are completing this tutorial from a public computer in Fondren Library and are logged on using the gistrain profile, follow the instructions below:

1. On the Desktop, double-click the Computer icon >  gisdata (\\file-rnas.rice.edu) (R:)  >  Short_Courses  >  Introduction_to_Coordinate_Systems_and_Projections .
2. To create a personal copy of the tutorial data, drag the Projections  folder onto the Desktop.
3. Close all windows.

OPTION 2: Accessing tutorial data online using a personal computer

If you are completing this tutorial from a personal computer, you will need to download the tutorial data online by following the instructions below:

1. Click Projections.zip above to download the tutorial data.
2. Open the  Downloads  folder.
3. Right-click  Projections.zip  and select Extract All...
4. In the 'Extract Compressed (Zipped) Folders' window, accept the default location into the Downloads folder and click Extract.
5. Drag  the  Projections  folder onto your Desktop.
6. Close all windows.
   

Exploring GIS Data Coordinate Systems

Connecting to a Folder

1. On the Desktop, double-click the ProjectionsTutorialData folder and open ProjectionsTutorialData.aprx.
2. In the Catalog pane to the right of the Map Window, right-click the Databases folder and select Add Database.
3. In the ‘Select Existing Geodatabase’ window, select the ProjectionsTutorialData.gdb geodatabase located in the ProjectionsTutorialData folder on the Desktop.
4. Verify that C:\Users\gistrain\Desktop\ProjectionsTutorialData/ProjectionsData.gdb is listed as shown below and click OK.

In the Catalog pane, under the Databases folder, you should now see a file path connecting directly to your ProjectionsTutorialData geodatabase.

Previewing GIS Data

1. In the Catalog pane, in the Databases folder, double-click to expand the ProjectionsData geodatabase.

Inside the ProjectionsData geodatabase, you will notice three feature classes.

1. In the Catalog pane, select and hover over the County feature class.

This feature class contains a polygon delineating the boundaries of Harris County. It was obtained from Esri’s now outdated Census 2000 TIGER/Line Data website at http://www.esri.com/data/download/census2000-tigerline.

1. In the Catalog pane, select and hover over the MajorRoads feature class.

This feature class contains centerlines of all of the major roads within the greater Houston and Harris County region. It was obtained from the COHGIS 2011 route datasets link on the City of Houston GIS (COHGIS) 2011 Datasets website at http://mycity.houstontx.gov/home/cohgis.html.

1. In the Catalog pane, select and hover over the Schools feature class.

This feature class contains the point locations of all schools in Texas. It was obtained from the Download Schools link on the Texas Education Agency (TEA) Texas School District Locator – Data Download website at http://ritter.tea.state.tx.us/SDL/sdldownload.html.

All GIS data is created in a particular geographic coordinate system used for mapping locations on a three-dimensional sphere, based on measurements of latitude and longitude. In addition, much GIS data has already been projected onto a two-dimensional plane, based on linear measurements, such as feet or meters. In order for ArcGIS to position a data layer in its proper geographic location, it must be told the proper coordinate system and projection in which the data was originally created. Otherwise, the data layer can be displayed as an image of sorts, but the image will not necessarily line up correctly with other data layers in the Map Window.

Determining if Layers are Defined

In some cases, the geographic coordinate system and projection of the data has already been defined in ArcGIS, while, in other cases, you will need to define it yourself based on metadata obtained from the source of the data. First, you will need to determine whether or not each feature class has already been defined.

1. In the Catalog pane, right-click the MajorRoads feature class and select Add to New Map.
2. When the Map loads, right-click on MajorRoads in the Contents pane and select Properties.
3. In the left menu, click the Source tab. Scroll down and expand the Spatial Reference section.

Under ‘Spatial Reference’, scroll down and notice that both a geographic coordinate system and a projection are listed, which indicates that the coordinate system of this feature class has already been defined.

Note that the geographic coordinate system is GCS North American 1983. “GCS” stands for Geographic Coordinate System. This particular coordinate system is more commonly referred to as the North American Datum 1983 or NAD83.

The general type of projection is Lambert Conformal Conic, but its properties (central meridian, standard parallels, latitude of origin, etc.) have been modified to be optimal for South Central Texas. These modifications are saved under the Projected Coordinate System name NAD 1983 StatePlane Texas South Central FIPS 4204_ Feet. The State Plane projection system divides each state into multiple planes that are suitable for mapping local regions within each state. Houston falls within the South Central plane in Texas.

1. In the ‘Layer Properties: MajorRoads’ window, click Cancel.
2. In the Catalog pane, right-click the County feature class and select Add to Current Map.
3. When it is added to the Contents pane, right-click on the layer and select Properties.
4. In the left menu, click the Source tab. Scroll down and expand the Spatial Reference section.

Note that the spatial reference is in an Unknown Coordinate System, meaning that the coordinate system of this feature class has not been defined. In other words, ArcGIS does not know which coordinate system the data was created in and, therefore, will probably not be able to accurately line up this data layer with other data layers. Before you can reliably use this feature class, you will need to define its coordinate system and projection.

1. In the ‘Layer Properties: County’ window, click Cancel.
2. In the Catalog pane, right-click the Schools feature class and select Add to Current Map.
3. When it is added to the Contents pane, right-click on the layer and select Properties.
4. In the left menu, click the Source tab. Scroll down and expand the Spatial Reference section.

Note that the Schools feature class uses the same geographic coordinate system as the MajorRoads feature class: GCS North American 1983. Even the general projection type is the same: Lambert Conformal Conic. However, its properties have been modified to be suitable for all of Texas, rather than just the South Central portion of Texas, so the Projected coordinate system name is now Texas Centric Mapping System/Lambert Conformal. The Texas Centric Mapping System is an example of a statewide projection that is suitable for mapping the entire state. Statewide projections are commonly developed for large or elongated states, such as Alaska, California, Michigan, and Texas, which would otherwise be divided into numerous small bands using the State Plane system.

1. In the ‘Layer Properties: Schools’ window, click Cancel.

The geographic coordinate system and projection for each of the three feature classes are summarized in the table below.

Displaying Undefined Layers

You will now see what happens when you attempt to add an undefined layer to ArcGIS. The data frame, or display, takes on the coordinate system and projection of the first layer added to it. In this case, since the MajorRoads layer was the first layer added, the data frame will take on the State Plane Texas South Central projection. To verify this, you will check the coordinate system of the data frame.

1. In the Contents pane, right-click on Map and select Properties.
2. In the menu on the left, select Coordinate Systems.

Notice that, indeed, the State Plane Texas South Central projection is listed.

1. In the ‘Map Properties: Map’ window, click Cancel.
2. In the Contents pane, right-click on the MajorRoads layer and select Zoom to Layer.

Notice that both the MajorRoads layer and the Schools layer appear to line up properly, even though they have different projections. This is because ArcGIS is able to perform “on-the-fly” projection, as long as the coordinate systems of both layers are defined. On-the-fly projection means that the Schools layer is being superficially displayed in the State Plane Texas South Central projection of the data frame, based on calculations performed by ArcGIS; however, the projection of the Schools feature class itself has not been altered from the Texas Centric Mapping System projection.

Though not recommended, on-the-fly projection may be used whenever a visual display of data is all that is required. On-the-fly projection should never be used when attempting to perform calculations between two layers involving measurements of distance or area, as these measurements are most accurate when all data layers are in the same projection.

Now you will see how ArcGIS handles undefined layers.

1. In the Contents pane, right-click on the County layer and select Zoom to Layer.

Notice that the County layer does not appear on where it should on the Map. If you zoom out enough, you will see that it is in the middle of the Pacific Ocean This is because ArcGIS does not know where to locate the County layer relative to the other layers, since it is not defined. Instead, ArcGIS does the best it can and treats the coordinates of the County layer as if they were in Texas South Central State Plane coordinates, even though they are not.

Notice when you move your cursor around the map display, the coordinates listed in the bottom right of the window are similar to those shown below, indicating units of millions of feet.

1. In the Table of Contents, right-click the County layer and select Zoom to Layer.

It is evident that both layers are being drawn in the data frame, but ArcGIS simply does not know how to line them up properly. Notice when you move your cursor around the map display now, the coordinates are similar to those shown below, indicating units around -95 and 29 feet.

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Whenever you come across X-coordinates between -180 and 180 and Y-coordinates between -90 and 90, you can be fairly sure that they are actually geographic coordinates of latitude and longitude listed in decimal degrees. The fact that Houston is located at 95° W, 29 N° is further confirmation that the county layer was created in a geographic coordinate system, rather than a projection. In this case, ArcGIS is treating these measurements of decimal degrees as if they were measurements of feet in the State Plane Texas South Central System, with the end result of locating Harris County in the middle of the Pacific Ocean.

1. Right-click the MajorRoads layer and select Zoom to Layer.

Whenever you encounter this type of situation, where two layers show up in the Contents pane, but do not line up as you expect them to, you know it is most likely a projection problem.

Determining projections for undefined layers

Of the three feature classes you examined, the County feature class was the only one that was undefined. Since the spatial reference information cannot be located using ArcGIS, you must turn to the source of the data itself. The data was originally downloaded from the following website: http://www.esri.com/data/download/census2000-tigerline. A screenshot of the metadata provided on that website appears below.

The metadata indicates that you must define the geographic coordinate system for the layer as NAD83, or North American Datum 1983. Fortunately, this is the same geographic coordinate system used by the other two layers. Since no projection is listed, you can assume that the layer has not been projected and was created in a geographic coordinate system only. The fact that the units are listed as decimal degrees further confirms this assumption.

Defining Undefined Layers

Now that you know the geographic coordinate system of the County feature class, you will need to define it so that ArcGIS knows, as well. Please note that the geographic coordinate system and projection must be defined based on metadata provided from the data source itself. If you cannot locate such metadata, you should contact the provider of the data for such information. You cannot simply guess or decide which projection to use when defining the data (though, occasionally, you may have to resort to educated guesses and trial-and-error when metadata absolutely cannot be tracked down.)

1. In the Analysis pane, click the Tools button.
2. In the Geoprocessing pane to the right of the Map Window, in the Find Tools search bar, type ‘Define Projection.’ Select Define Projection (Data Management Tools).
3. In the ‘Input Dataset or Feature Class’ drop-down menu, select County.
4. Click the Atlas button to the right of the Coordinate system drop-down menu.
5. In the Coordinate System window, expand Geographic coordinate system > North America > USA and territories > NAD 1983 (2011). Click OK.
6. Ensure your Geoprocessing pane looks like this pane and select Run.

Notice that the County layer has been projected onto the correct location of Harris County.

1. In the Catalog pane, right-click the County layer and select Properties. Select the Source tab and expand Spatial Reference.

Notice that the Geographic coordinate system is now listed, since you have defined the feature class.

1. In the ‘Layer Properties: County’ window, click Cancel.

If you are only interested in creating a visual map, as you have done now, then relying upon on-the-fly projection will suffice. If, on the other hand, you are interested in performing any sort of calculations between the layers that rely upon distance, area, or spatial overlap, then it is recommended that you use the same projection for all layers. In addition, you can only measure linear distances in a projected coordinate system, not in a geographic coordinate system, so the County layer would need to be projected anyway, before any sort of measurements could be made.

Selecting a Map Projection

Now that all of your data has been defined, you can transform all the feature classes into a common projection. Unlike the process of defining data, where there is only one projection you can use (the one specified in the metadata), when projecting data, you can select whichever projection best suites your particular combination of data and tasks at hand. Since your data focuses on the Harris County region, the State Plane Texas South Central projection would be suitable.

Determining which layers to project

The State Plane Texas South Central projection has been selected for the final map projection.  All layers that are not currently in that projection will need to be re-projected, such as the County and Schools feature classes. All layers that are already in the final projection can be left as is and do not need to be re-projected, such as the MajorRoads feature class.

Projecting Layers

1. In the Geoprocessing pane, click the Back arrow.
2. In the Find Tools search bar, type ‘Project’. Select Project (Data Management Tools).
3. For the Input Dataset or Feature Class drop-down menu, select County.

Note that the output dataset is saved to the same geodatabase as the input dataset and the name of the output feature class has the suffix “_Project” tacked onto the end of the original input feature class name.

1. Next to the ‘Output Coordinate System’ drop-down menu, click the Atlas button.

While you could click navigate to or search for the State Plane Texas South Central coordinate system yourself, in this case, you know that the same coordinate system is already used by the MajorRoads layer. In such an instance, it is easier to import the coordinate system from another layer, especially if you are not familiar with where the coordinate system of interest is stored.

1. In the Coordinate System window, double-click Layers.
2. Since MajorRoads is in NAD 1983 StatePlane Texas South Central FIPS 4204 Feet, it is listed underneath Layers. Single-click it and select OK.
3. Ensure that your Geoprocessing pane appears as shown below and select Run.

When previously defining a layer, you simply updated the metadata attached to that layer to include the known coordinate system and projection. When projecting a layer, you have actually created a second copy of the data in the new coordinate system. The original layer stays in the native coordinate system and the new layer is converted to the desired coordinate system.

1. In the Catalog pane, right-click County_Project and select Properties. Navigate to the Source menu and expand Spatial Reference.

Note that the Geographic coordinate system is still GCS North American 1983 and that the new coordinate system is NAD 1983 StatePlane Texas South Central FIPS 4204 Feet.

1. In the ‘Feature Class Properties’ window, click Cancel. In the ‘Layer Properties: County_Project’ window, click Cancel.

You will now repeat the same process to re-project the Schools feature class from the Texas Statewide Mapping System projection into the State Plane Texas South Central projection.

1. Repeat steps 3-7 with the Schools layer. When your Geoprocessing pane looks like the one shown below, select Run.

All of your layers have now been projected into the State Plane Texas South Central projection.

1. In the Contents pane, right-click the Schools layer and select Remove.
2. In the Contents pane, right-click the County layer and select Remove.

Since all layers in your data frame are now in the same projection, you could proceed to make accurate calculations of distance and area between the layers.

Last updated August 14, 2018.