Image Library georeferencing tutorial
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.
I. Defining your objective
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.
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:
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