• The process of georeferencing involves placing an image (e.g., scanned photo or map) in real-world coordinates.  This involves telling the computer where certain portions of the image are in relation to known locations on the Earth's surface.  Usually, the technician will use a digital database map of roads, rivers or some other known and easily identifiable map to georeference (or rubber-sheet) the image.  Once your image is georeferenced you will then be able digitize its data into “real-world” coordinates.
• The purpose of digitizing is to input spatial data into a GIS so that it may be utilized.  This involves transferring data (both spatial and attribute) from a map or photograph, or other image type into digital form.  There are two basic types of digitizing:  (1) heads-up, and (2) heads-down.  Heads-up digitizing occurs when you trace over a scanned image in the computer.  Heads-down occurs when you use a digitizer tablet and trace over a paper or myler copy of the map.  Both types are useful, but require different preparations, processing, and experience.  This tutorial will instruct you to perform heads-up digitizing.
  

1. Open ArcView from either the Start button or by double-clicking on the desktop icon.
2. Go to the File pull-down menu and select Extensions.  Go down and put a checkmark next to the Image Analysis Extension (example 14, example 15).
3. Open a new view by selecting the New button and add any theme that will be used as a reference theme by clicking the add theme button (it looks like a plus sign below the Edit menu (example 1, example 2, example 3, example 4).  Possible themes include country.shp, states.shp, cities.shp  These will be located in the directories under  --    c:\esri\esridata\  Consult with Dr. Lambert as to which themes would be best for your project.
4. Click once on the legend for the reference themes you have chosen and go to the Theme pull-down menu and select Hide/Show Theme (example 5).  This makes their legends less “bulky” in the legend sidebar.
5. Double click the legend editor (just double-click on the name of the theme in the legend) for the themes you will be using for reference and change their outlines to red or green or blue and their foregrounds to transparent (example 6, example 7,  example 8,  example 9,  example 10,  example 11,  example 12,  example 13).
   

1. Once you have loaded the appropriate reference theme in the view it is possible to perform the rubber-sheeting process.
2. Zoom in as much as possible to the area of interest.  However, make certain that you can still see all of the area that will ultimately be covered by the image you are about to align (example 16, example 17).
3. Now add the image file that you wish to rubber-sheet by going to clicking the add theme button (the little plus sign below the Edit pull-down menu) (example 18, example 19, example 20).
4. Then drag the reference theme on top of the image theme in the legend sidebar (example 21).
5. Make sure that image theme is active.  Click the align button once (it looks like a black square with an arrow pointing to the upper right), this will place the image behind the reference theme so you can begin the rubber-sheeting process (example 22)
6. Begin aligning the image by clicking once on a distinctive location in the image and then once on the similar location on one of the reference themes.  You wil notice that the image will “jump” to the new location (example 23).
7. Repeat this process with another distinctive location elsewhere on the image (example 25, example 26, example 27).  Make certain to always click the feature on the image first and then on the respective location on the reference themes.  That way ArcView will know which way you wish to move the image.
8. Once the image is aligned sufficiently well you are ready to save it under a new name.  Keep in mind, that the alignment process is rarely perfect and your new stretched image may not exactly match the reference themes (example 28).
9. Once the image is aligned properly, go to the View pull-down menu and select Save Image As (example 29, example 30, example 31).  Give it a short but meaningful name and save it your folder.
10. You have now georeferenced the image.  It has geographically correct coordinates.  It is also ready to be used in the next step of this process.
    

1. Now you will begin digitizing a theme (layer) based on your scanned image.  This theme could be the locations of musical instrument usage, or incidence of blue ceramic tiles, or linguistic patterns (whatever your image shows).  You must start by creating a new theme (do this by clicking New Theme in the View pull-down menu).  If you were mapping where blue ceramic tile artifacts are found you would most likely choose a POINT Theme.  If you were mapping linguistic patterns you would most likely choose a POLYGON Theme.  If you were mapping old trade routes you would most likely choose a LINE Theme.  It would useful to discuss your theme types with Dr. Lambert during the design phase of your project.
2. Once you have determined what type of theme you will create (POINT, LINE, or POLYGON) you will be ready to digitize.  Go to the View pull-down menu and select New Theme (example 32).  Navigate to your folder on the hard drive and give it a meaningful name.  You may also need to create data for the attribute table of this new theme (example 33, example 34).  The attribute table contains any kind of data that you think is appropriate for the theme.  It contains fields or columns of data.  The fields contain any tabular information you which to include relating to the features on the map.  For example, the blue ceramic tile theme might have fields called “Descriptions,” "Locations,” and "Time Period.”
3. If you determine, upon consultation with Dr. Lambert, that you will be using attribute tables you will need to set them up.  Do this by opening its Table under the Theme pull-down menu.  Then by selecting the Add a Field option from the Edit pull-down menu.  Make it a String data type, and make it at least 60 characters wide.
4. Using the techniques demonstrated by Dr. Lambert begin digitizing the data from scanned image (example 35, example 36).  As you complete a feature enter its description in the appropriate attribute fields.  Hit Enter, then return to the map to begin digitizing the next feature.
5. Once you have digitized all of the features that your chose and are satisfied with their quality save the Theme.  You may need to perform a clip operation if you have been digitizing polygons on the map.  Click here to learn how to clip.

Last updated on June 8, 2004