Embedded Files
Before tackling this tutorial, you will need to download and install a dataset following these instructions:
Create a folder called Projections somewhere under your personal directory (e.g. C:\Users\jdoe\Documents\Tutorials\Projections\).
Download the data for this exercise then extract the contents of Projections.zip into your newly created Projections folder.
Open the map project
Open the map project
Open the projection.aprx file.
The map is composed of three layers: a Deer Wintering Areas layer, a Maine counties layer and a hillshade raster layer. All layers seem to cover the same spatial extent yet, as you will shortly learn, they do not necessarily need to share the same underlying coordinate system.
Identify a layer’s coordinate system
Identify a layer’s coordinate system
Right click on the Counties layer and select Properties.
In the Layer Properties window, select the Source tab .
Expend the Spatial Reference tab.
The Spatial Reference block indicates that the Counties layer is defined in a UTM NAD 1983 (zone 19N) projected coordinate system (PCS). The PCS was built off of a GCS North American 1983 geographic coordinate system (GCS). It's important to remember that all PCSs are built off a GCS. So the fact that both a PCS and GCS definition are present in the Spatial Reference properties does not mean that the data layer's coordinates are stored in both a PCS reference system and a GCS reference system. They are in fact stored in a PCS reference system.
Other useful information that can be extracted from the Spatial Reference properties is the layer's coordinate value's unit (meters in this example). You'll also note the presence of a WKID number, 26919 and its associated authority, EPSG. EPSG is one of several accepted spatial reference systems used to standardize coordinate system definitions. These standards facilitate the transfer of coordinate system information from one software application to another.
Layers do not need to share the same projection in a single map document. ArcGIS converts projections on the fly to a common map projection defined by the Map frame.
Click Cancel to dismiss the Counties' Layer Properties window.
Right click on dwa layer and select Properties.
In the Layer Properties window, select the Source tab .
Expend the Spatial Reference tab.
The information displayed under Spatial Reference indicates that the Deer Wintering Area’s coordinate system has no projected coordinate system (PCS), it is solely based on a Geographic Coordinate System (GCS) that is defined by a WGS 1984 datum.
Click Cancel to dismiss the properties window.
Likewise, explore the coordinate system associated with the hillshade raster layer.
The hillshade addopts the same GCS as that of the deer wintering area layer (dwa).
Identify the Map’s coordinate system
Identify the Map’s coordinate system
The map frame's coordinate system is the coordinate system (CS) used to display the layers. It does not need to share the same CS as any one of those used in the different layers. Next, you will learn how to identify a Map's CS.
Right click on the Map in the Contents pane and select Properties.
In the Map Properties window window, select the Coordinate Systems tab.
The properties window indicates that the coordinate system is NAD 1983 UTM Zone 19N. To get more coordinate system details, click on the Details link.
The information indicates that the Map displays all layers in UTM Zone 19 North (NAD83)—the same projection as the one used by the Counties layer.
Note that a even though a Map’s coordinate system does not need to match that of any one of the data layers, it is usually good practice to ensure that all layers in a map share the same coordinate system when possible. In this exercise, we will make UTM zone 19 N (NAD83) this project’s default coordinate system.
In the next step, you will re-project the Deer Wintering Area’s layer to a projection that matches that of the Counties' and the map document's reference system.
Projecting a vector layer to a new coordinate system
Projecting a vector layer to a new coordinate system
In the Analysis tab, click on the Tools button.
In the Geoprocessing search bar type project.
The first link should be the Project tool. Click on that link to bring up its geoprocessing window.
Note that the Project tool can only be used with vector layers. A separate tool, Project Raster, is used for raster layers and will be highlighted later in this tutorial.
Select dwa as the input feature class.
Name the output dwa_UTM.shp. Be sure to save it in the current project folder and not in a Geodatabase.
Since we'll be adopting the Counties' coordinate system, we can select Counties from the Output Coordinate System pull down menu. Doing so will display its PCS in the field box.
Alternatively, you could have clicked on the Globe icon next to the pull-down menu and selected the NAD 1983 UTM Zone 19N CS from the list of options.
You'll note that a geographic transformation option appears in the last field of the geoprocessing tool. Whenever a layer is projected to a new projection that does not share the same underlying GCS (and by extension, the same underlying datum), the software will need to transform the underlying latitude and longitude values from one GCS to another before attempting a re-projection. The tool allows you to specify the GCS transformation method. There can be many methods to choose from. While this topic goes well beyond the scope of this tutorial, bear in mind that if you are seeking high accuracy in your coordinate values, you will need to pay attention to the GCS transformation method you choose. In this tutorial, we will accept the default geographic transformation method.
Click Run to execute the geoprocess.
If the layer is not automatically added to the Contents pane, add it.
Remove the original dwa layer from the Contents pane (right-click on the layer and select remove) since we do not need two copies of the same set of features.
Projecting a raster layer to a new coordinate system
Projecting a raster layer to a new coordinate system
The approach we'll take to re-project the hillshade raster layer is the same, however, we will need to use a different geoprocessing tool. The project tool used in the last step is only used for vector data. For a raster dataset you will need to use the project raster geoprocessing tool.
In the Geoprocessing search bar type project raster.
The first link should be the Project Raster tool. Click on that link to bring up its geoprocessing window.
Select hillshade as the input raster.
Name the output hillshade_UTM.tif. Be sure to save it in the current project folder (and not in the Default.gdb geodatabase) and to explicitly type the .tif extension (if no extension is typed, the output will default to the archaic ArcInfo grid format).
If you chose to save it in a geodatabase, remove the .tif extension from the name.Since we'll be adopting the Counties' coordinate system, select Counties from the Output Coordinate System pull down menu.
Note that as was the case in the last step, a GCS transformation will be needed. We'll stick with the default transformation method provided by the tool.
Click Run.
Since a raster is a grid object, a re-projection is likely to reshape the grid to fit the new CS. This may require that output cells be computed from more than 1 input cell value in which case you have the option to specify how to resample the input cells to generate the one output cell. For this example, we'll stick with the Nearest Neighbor default.
You also have the option to specify the output cell size. Recall that you are going from a lat/long coordinate system where the pixel size is specified in degrees, to a projected coordinate system where the pixel size is specified in meters. ArcGIS Pro will suggest a cell size. You can, however, opt for a different output cell size calculation in the Output Cell Size option box. For this example, we stick with the default pixel size suggested by the tool.
Once the new raster is added to your Contents pane, you might need to change its symbology to look like that of the original raster layer.
Select the newly created raster layer, then click on the Raster Layer tab.
In the Raster Layer ribbon, choose Stretch from the Symbology pull-down menu, then select minimum-maximum from the Stretch Type pull-down menu.
You can fine tune the color scheme by "stretching" the range of colors.
In the Raster Layer ribbon, click on the Stretch icon and select Minimum Maximum from the pull-down menu.
Remove the original Hillshade layer. You should now be left with three layers, all in a UTM NAD 1983 19N projected coordinate system.
Defining coordinate system for a GIS file
Defining coordinate system for a GIS file
Up to now all layers accompanying the map document had their coordinate system properly defined. However, there may be times when coordinate system information is missing or incorrect. In the next step, you will learn how to define a coordinate system for a layer with missing coordinate system information..
Add the layer conserved_lands.shp to the map.
So where is the conserved_lands layer in the map view?
Click on the Full Extent icon in the Map ribbon . This will zoom the map to the full extent covered by all layers in the Contents pane.
The new layer is clearly way off target. So what happened?
Spatial reference information embedded in a layer tells ArcGIS what reference system was used to record XY or lat/long values for that layer. If the reference system information is missing, the best ArcGIS can do is assume that the layer shares the Map’s spatial reference system. This assumption clearly does not work in our case. So we need to properly define the new layer’s coordinate system.
It turns out that the layers coordinate system is in a USA Contiguous Albers equal area conic projection. We will therefore need to define the coordinate system for the layer using the Define Projection tool.
Type Define Projection in the geoprocessing search bar.
Click on Define Projection (Data Management). This should be the first search result in the list.
Select conserved_lands as the input feature class.
Click on the select coordinate system icon to the right of the field. Then, select Projected Coordinate Systems >> Continental >> North America >> USA Contiguous Albers Equal Area Conic .
Click OK.
Click Run to execute the geoprocess.
When the process is complete, the conserved_lands layer should be properly stacked on top of the other existing layers.
Note that the Define Projection tool can also be used with raster datasets.
Re-project the conserved land layer
Re-project the conserved land layer
To match the Map's coordinate system (as well as that of the other layers in the map) we will now re-project the Conserved_lands layer to the UTM coordinate system.
On your own, re-project conserved_lands to UTM NAD 83 Zone 19 N following the steps outlined earlier in this exercise. Name the newly projected layer conserved_lands_UTM.shp and save it under the current workspace. Add it to the map when done.
Note that a datum transformation is not necessary here since both the Albers and UTM projections work off of the same NAD 1983 GCS/datum.
Finally, remove the original conserved_lands layer from the TOC. You should be left with four layers, all sharing the same PCS. This wraps up this tutorial.
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