Maps downloaded from the Image Library can be used in desktop GIS applications that are capable of handling MrSid imagery. Most images on this site are produced by scanning a copy of the original paper map.These images are not properly georeferenced or GIS ready. It is, however, possible to use the imagery in a GIS as a base layer or thematic overlay.

This tutorial describes how to import a map image into ESRI 's ArcGIS package and align the map with other spatial data layers. The sample dataset for this tutorial can be downloaded and used to help you follow along with the steps in this tutorial.

Download sample data set (georef.zip 11.8MB) for the Geologic Map of the Spirit Lake East Quadrangle, Skamania County, Washington by R.C. Evarts and R.P. Ashley. This data set includes local roads, streams, lakes, and points of interest, along with a DEM hillshade of the Spirit Lake East 24K quadrangle.

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I. Defining your objective

For this excercise, you have collected and prepared data in ArcGIS for the Spirit Lake East 24K quadrangle where you will be doing some field work over the next several weeks. You would like to produce a map like the one shown here that shows where the geologic contacts and formations exist in relation to the most recent US Forest Service roads, streams, and known points of interest from the Geographic Names database.

II. Preparing the data

It is important to note that before you set off to georeference your image, you will want to gather together the data you plan to include in your map and perform any necessary geoprocessing steps. Most importantly, the projection of your map should be defined so that you have a destination projection for your image. Furthermore, the data should be aligned properly. Once you have established a map projection, it is easier for you to overlay your geologic map image.

The sample data contain formatted GIS data and an ArcMap project file. These data layers have been projected to WGS_84 UTM zone 10. The data have also been clipped to the extent of the Spirit Lake East 24K quadrangle.

Step1: Extract the files in "georef.zip" to the directory where you plan to work on this tutorial.

Step2: Open the ArcMap project entitled "tutorial.mxd"

Your ArcMap Window opens with a simple map of the Spirit Lake East quadrangle boundary, roads, streams, and points of interest. The hillshade layer can be toggled on and off to reveal the elevation hillshade created from the 10 meter resolution dem

III. Importing the Geologic Map

You can now import the geologic map into ArcMap. The map is stored in MrSid format just as it were downloaded from the Image Library. Do not be alarmed that the map does not show up in your ArcMap window after you add the map. We will reposition it later.

Step3: Add the "SpiritLakeGeology.sid" file to ArcMap by clicking the Add data icon and choosing the file from the loaction where your sample data are located.

You may get a notification informing you that the data cannot be projected, but you can dismiss this because we plan to reference the layer . Because the geologic map has no referencing information it will not appear inline with your existing data layers, but there is a way to bring that map into the window so you can see it.

Step4: Open the Georeferencing Toolbar if it is not already opened. To do this, Right Click on the menu bar at the top of your screen to reveal the menu options. Choose "Georeferencing" from the list to open the menu.

Step5: From the Georeferencing Toolbar's pull down window, select the name of the geologic map MrSid image (SpiritLakeEast.sid). Under "Georeferencing" select "Fit to Display".

The entire scanned map, including the legend, will now appear in your ArcMap window, but it is not georeferenced. You now need to align the corners of the mapped geology with the corners of the Spirit Lake East quadrangle boundary.

IV. Aligning the Map

The Georeferencing toolbar contains the "Create Control Points" tool which is indicated by the button with the green and red crosshairs. This tool allows you to align one point on your raster image with a point on the map. The following image shows the circled set of points that you will align first.

To align these points you will zoom in close and select the points with the Georeferencing Control Points tool. Follow these steps to create a control point.

Step6: Zoom in very close to the NW corner of the geologic map.

Step7: Click the Control Points tool in the Georeferencing toolbar to activate the tool. Place the cursor over the corner of the geologic map and click once to place a green crosshair in line with the edge of the map.

Step8: Now, navigate to the corner of the quadrangle boundary. To do this you may have to activate the Zoom tool again and zoom out before zooming back in to the quadrangle corner. You will not lose your crosshair by switching to the zoom tool.

Step9: Reactivate the Control Points tool and click on the corner of the quadrangle boundary. This completes the linking procedure for this control point. the corner of your map should align itself with the NW corner of the quadrangle.

Repeat Steps 6 - 9 for the remaining 3 corners of your map. You will notice that the alignment is pretty good after the second link, but it is a good idea to have at least 3 control points. When you are finished, your geologic map image should be aligned properly with the rest of your data.

V. Save your work

Step10: From the Georeferencing toolbar choose "Update Georeferencing" and then click on "Delete Control Points" to remove the crosshairs from your map.

This step saves the georeferencing information for your MrSID image in a .sdw file (Sid World File). The file is created in the same folder as your image and must remain in the same folder as your image, with an identical base filename, in order for your sid image to be georeferenced.

The world file is simply a short text file that tells ArcGIS how many pixels in your image per degree of latitude or longitude, and where one corner of your image should be anchored. It looks something like this:

2.0300302353845714
0.0242687075860875990
0.0259324485857842200
-2.0333230225428656
566483.69886933011
5136631.5007790830

At this point, you are essentially finished. Congratulations! You will now be able to use this map along with other spatial data and plan for your upcoming fieldwork more effectively.

If you would like to learn more, the following section describes some other options you may want to explore.

VI. More Options

1. You have the option to "Rectify" the image by using the georeferencing toolbar. This step will generate a large Tiff file and Tiff world file (.tfw). This process takes a few minutes to run, but may be well worth it if you wish to crop the collar or other features from the map in a program like Photoshop.
   
   
2. To "drape" your geologic map over the hillshaded elevation layer, set the transparency of the geologic map to 30% - 40% (double click the layer in the legend to access its properties), and turn on the hillshade layer making sure that the map overlays the hillshade.
   
   
3. You may wish experiment with incorporating other facets of the scanned map into your GIS. Perhaps you wish to digitize the traverse lines (A-A') and hot link them to a cropped image of the cross section. You could also digitize formations and link them to a cropped image of the legend description if you wish. The following screenshot shows the sample data map with an overlay of the sketch map showing phaneritic intrusive rocks from the lower right corner of the map publication. The possibilities are endless!