Gereferencing in ArcGIS March 2017

A bit of background Development of Geographic Information Systems (GIS) goes back to the 1960s Currently an abundance of “GIS- ready” resources available to GIS users. However… What about geographically-related resources that were not specifically created for use in GIS? Many such resources pre-date GIS, while others continue to be produced. Can these still be used as part of a GIS model? Traditional Paper Maps Aerial Photographs Layout Drawings Satellite Images etc.

Yes, they can! (but there are a couple of requirements) Georeferencing will allow you to use materials that are not GIS-ready in a GIS model. Two basic prerequisites: Item must be in an image file that ArcGIS can handle Therefore, items such as paper maps or photographic prints will need to be scanned. Common image formats for ArcGIS include: BMP, JPEG, JPEG 2000, PNG, TIFF, etc. Image must be defined using an appropriate coordinate system/projection.

Coordinate System? Why? Suppose that you have a scan of a historical map of Tombstone, Arizona. ArcGIS will need you to answer a few questions.

Where does it go? Here? Here? Here?

How big an area of the map does it cover? This big? This big? This big?

How should it be oriented?

(2D representation of the earth) Datums & Projections Datum (3D frame of reference using latitude/ longitude coordinates to measure locations, as well as elevations at particular points on the earth) (converted to) Projection (2D representation of the earth) Source: https://commons.wikimedia.org/wiki/File:A_large_blank_world_map_with_oceans_marked_in_blue.svg

Projections Various types of projections are used to represent the 3-dimensional earth on a flat (2-dimensional) surface. typically tailored to a particular purpose or geographic area. Try to minimize distortion in Shape, Area, Distance or Direction (SADD) Examples: State plane projections used to represent various U.S. states. Source: https://alidade.wikispaces.com/Florida+SPCS+Zones

Q: How do you convey information about your image’s position, geographic area, and orientation to ArcMap? A: By defining the image using a suitable coordinate system/projection.

Which coordinate system/ projection should you use? Maps will sometimes indicate the coordinate system that was used in their creation. In this case, stick with the earlier coordinate system. In other cases, no such information is provided. For example, an aerial photograph would not typically be associated with a coordinate system. User must then make an educated assessment of what system to use, based upon factors such as the location of the image and how big a geographical area it covers.

Example No. 1: Georeferencing a Map For this example: The map indicates the coordinate system that was used to create it (North American Datum of 1983, a.k.a. NAD 83) Use this same system for the georeferencing The map includes lines of longitude and latitude The various intersections of these lines may be used for applying the coordinate system to position and orient the image.

Open a new GIS model and assign the coordinate system for the frame. Right-click on the name of the data frame and select “Properties…” In the Data Frame Properties window, go to the “Coordinate System” tab. Expand the lists of Geographic Coordinate Systems and Projected Coordinate Systems Find and select your desired system.

Load map image into model. If your Catalog window is not already open, open it using the pull-down menu at the top of the screen WindowsCatalog If you cannot access the directory containing the image, use the Connect to Folder icon to establish a connection to the directory. Click-and-drag the image file into the mapping area from the Catalog window. You may get a message regarding missing spatial reference information. Click on “OK”

Activate the Georeferencing toolbar Right-click on the grey area at the top of the page. Select “Georeferencing” Click-and-drag on the header of the Georeferencing toolbar to move it to a convenient location (for example, at the top of the page).

Add Control Points On the Georeferencing toolbar, click on the Viewer icon to see the entire image. Use the Zoom In icon to zoom in on an intersection of a longitude line and a latitude line on the map. On the Georeferencing toolbar, click on the Add Control Points icon. Click on the desired point on the map. Right-click and select “Input DMS of Long and Lat…”. Enter the longitude and latitude of the point and click on “OK”.

Control Point Guidelines More control points allows a better final result. Distribute your control points throughout your image Don’t cluster all of your control points in one area. Target your control points at locations that are expected to remain constant. For example, Longitude/Latitude intersections on maps USGS survey markers Buildings, roads, and other fixed structures If georeferencing an aerial or satellite image, use the point where the structure intersects the ground rather than a point higher up on the structure. Warning: Natural features, such as rivers/streambeds and coastal features (such as sandbars) can shift over time, making them potentially unreliable as control points.

Checking your “fit” To see how well your control points are defining the area of your image, click on the View Link Table icon on the Georeferencing toolbar. Generally desirable to achieve low residual values. Points with high residual values can be deleted and replaced with new points. Greater numbers of points allow more accurate transformations (for example, a 2nd order polynomial will allow a more accurate transformation than a 1st order polynomial)

Update your Georeferencing and check fit against a background map. At the left end of the Georeferencing toolbar, expand the “Georeferencing” dropdown menu and select “Update Georeferencing”. Use the dropdown menu for the Add Data icon to add a basemap to your model. Adjust your map view to see how well your image matches the basemap. Might need to adjust transparency of your top map layer to check alignment. To adjust transparency, Open the Layer Properties Go to the “Display” tab Enter a new value for the layer transparency (higher values result in greater transparency).

Example No. 2: Georeferencing an Aerial Photo For this example: The photo is not associated with any coordinate system Therefore, the user will need to select an appropriate coordinate system. The map does not include lines of longitude and latitude To add control points, the user will therefore need to match landmarks in the image against their locations on a georeferenced reference layer. In the photo, road intersections provide recognizable landmarks. We will therefore use a USGS shapefile, which also contains roads, as our reference layer. This USGS shapefile is already georeferenced as a North American Datum 1983 HARN projection. The North American Datum 1983 HARN projection used in the reference layer should also be accurate for the area represented in the photo. Therefore, we will use this same projection for our data frame.

Open a new GIS model and assign the coordinate system for the frame. Right-click on the name of the data frame and select “Properties…” In the Data Frame Properties window, go to the “Coordinate System” tab. Expand the lists of Geographic Coordinate Systems and Projected Coordinate Systems Find and select your desired system.

Load reference layer and aerial photo into model. If your Catalog window is not already open, open it using the pull-down menu at the top of the screen WindowsCatalog If you cannot access the needed directory, use the Connect to Folder icon to establish a connection to the directory. Click-and-drag the reference layer and aerial photo into the mapping area from the Catalog window. For your aerial photo, you may get a message regarding missing spatial reference information. Click on “OK”

Bookmark area corresponding to your aerial photo Zoom and pan your reference layer until you are viewing an area slightly larger than that shown in your aerial photo. Bookmark this view for future use. Using the dropdown menu at the top of the screen, BookmarksCreate Bookmark…(assign name)OK

Activate the Georeferencing toolbar Right-click on the grey area at the top of the page. Select “Georeferencing” Click-and-drag on the header of the Georeferencing toolbar to move it to a convenient location (for example, at the top of the page).

Prepare to create control points. On the left side of the Georeferencing toolbar, expand the Georeferencing dropdown menu and select “Fit to Display”. To ensure that your reference layer is zoomed in to an acceptable extent, call up the view that you bookmarked earlier. In the dropdown menu at the top of the page, Bookmarks(name of bookmark)

Add Control Points Use the Zoom In icon to zoom in on the area in which you wish to create your control point. On the Georeferencing toolbar, click on the Add Control Points icon. Click on the desired landmark on your aerial photo. Click on the point on your reference layer where your landmark would be located. Zoom out (or call up your bookmark view) as needed to determine where to place your next control point.

Checking your “fit” To see how well your control points are defining the area of your image, click on the View Link Table icon on the Georeferencing toolbar. Generally desirable to achieve low residual values. Points with high residual values can be deleted and replaced with new points. Greater numbers of points allow more accurate transformations (for example, a 2nd order polynomial would allow a more accurate transformation than a 1st order polynomial)

Update your Georeferencing and check fit against a background map. At the left end of the Georeferencing toolbar, expand the “Georeferencing” dropdown menu and select “Update Georeferencing”. Use the dropdown menu for the Add Data icon to add a basemap to your model. Adjust your map view to see how well your image matches the basemap. Might need to adjust transparency of your top map layer to check alignment. To adjust transparency, Open the Layer Properties Go to the “Display” tab Enter a new value for the layer transparency (higher values result in greater transparency).

Exercises Two practice exercises are provided with this workshop. To access the exercises, Log on to any of the five GIS workstations located along the west wall of the Digital Scholarship Lab. Once you are logged in, go to the following directory: C:\Georeferencing

Exercise No. 1 Georeferencing a Map/Chart In the C:\Georeferencing directory, double-click on the “map_exercise.mxd” file. This model has already been set to the appropriate coordinate system. The model includes an image of a navigation chart that the user will need to georeference. When selecting control points for georeferencing, the user is encouraged to use intersections of longitude and latitude lines shown on the chart. The following slide can be used for reference when entering the longitude and latitude values for each control point.

Major Longitude and Latitude Lines

Activate the Georeferencing toolbar Right-click on the grey area at the top of the page. Select “Georeferencing” Click-and-drag on the header of the Georeferencing toolbar to move it to a convenient location (for example, at the top of the page).

Add Control Points On the Georeferencing toolbar, click on the Viewer icon to see the entire image. Use the Zoom In icon to zoom in on an intersection of a longitude line and a latitude line on the map. On the Georeferencing toolbar, click on the Add Control Points icon. Click on the desired point on the map. Right-click and select “Input DMS of Long and Lat…”. Enter the longitude and latitude of the point and click on “OK”.

Some basic control point guidelines In order to georeferenced an image, ArcMap needs a minimum of 3 control points. However, defining additional control points will allow a more accurate georeferencing. As much as possible, distribute your control points throughout the area of the image. This will also contribute to greater accuracy when georeferencing.

Checking your “fit” To see how well your control points are defining the area of your image, click on the View Link Table icon on the Georeferencing toolbar. Generally desirable to achieve low residual values. Points with high residual values can be deleted and replaced with new points. Greater numbers of points allow more accurate transformations (for example, a 2nd order polynomial will allow a more accurate transformation than a 1st order polynomial)

Update your Georeferencing and check fit against a background map. At the left end of the Georeferencing toolbar, expand the “Georeferencing” dropdown menu and select “Update Georeferencing”. Use the dropdown menu for the Add Data icon to add a basemap to your model. Adjust your map view to see how well your image matches the basemap. Might need to adjust transparency of your top map layer to check alignment. To adjust transparency, Open the Layer Properties Go to the “Display” tab Enter a new value for the layer transparency (higher values result in greater transparency).

Exercise No. 2 Georeferencing an Aerial Photo In the C:\Georeferencing directory, double-click on the “Aerial_photo_exercise.mxd” file. This model has already been set to the appropriate coordinate system. The model includes an image of an aerial photo and a reference layer showing Brevard County roads. When selecting control points for georeferencing, the user is encouraged to use road intersections visible in the aerial photo. On the following slide, suggested control points are marked by red four-pointed stars.

Suggested Control Points

Activate the Georeferencing toolbar Right-click on the grey area at the top of the page. Select “Georeferencing” Click-and-drag on the header of the Georeferencing toolbar to move it to a convenient location (for example, at the top of the page).

Prepare to create control points. On the left side of the Georeferencing toolbar, expand the Georeferencing dropdown menu and select “Fit to Display”. To ensure that your reference layer is zoomed in to an acceptable extent, call up the bookmarked “South Merritt Island” view. In the dropdown menu at the top of the page, BookmarksSouth Merritt Island

Add Control Points Use the Zoom In icon to zoom in on the area in which you wish to create your control point. On the Georeferencing toolbar, click on the Add Control Points icon. Click on the desired landmark on your aerial photo. Click on the point on your reference layer where your landmark would be located. Zoom out (or call up your bookmarked “South Merritt Island view) as needed to determine where to place your next control point.

Checking your “fit” To see how well your control points are defining the area of your image, click on the View Link Table icon on the Georeferencing toolbar. Generally desirable to achieve low residual values. Points with high residual values can be deleted and replaced with new points. Greater numbers of points allow more accurate transformations (for example, a 2nd order polynomial would allow a more accurate transformation than a 1st order polynomial)

Update your Georeferencing and check fit against a background map. At the left end of the Georeferencing toolbar, expand the “Georeferencing” dropdown menu and select “Update Georeferencing”. Use the dropdown menu for the Add Data icon to add a basemap to your model. Adjust your map view to see how well your image matches the basemap. Might need to adjust transparency of your top map layer to check alignment. To adjust transparency, Open the Layer Properties Go to the “Display” tab Enter a new value for the layer transparency (higher values result in greater transparency).

Questions?

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