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2D Maps - TerraServer Aerial Photo
and Map Images

*** Back to 2D Maps for Resource Management ***

 

This section focuses on working with images that can be used for base maps. Aerial photographs, topographic maps or other geographic images that can be used as a guide are generally the starting point for creating an new map.

  1. TerraServer Aerial Photo and Map Images
  2. Other Digital Image Files
  3. Geo-Referencing Aerial Photos that are Scanned for Base Maps

 

TerraServer Aerial Photo and DRG Images

As described in Basic Map Data, Microsoft's TerraServer is an excellent source of free US North American Digital Ortho Photos (DOP) and USGS topographic maps (Digital Raster Graphics or DRG). TerraServer is simple to navigate, and you can use a town name, street address or geographic coordinates to find the photos you want. The process works quickly with a cable or DSL connection. Terra Server has a compass rose (left) that you can click to shift the initial image to the place you want to see. You can also zoom in (which changes the resolution) or change the size of the image.

Rather than view TerraServer images individually, you might want to use a free program named USAPhotoMaps. The program will automatically download and merge multiple DOPs. You will need a broadband Internet connection for it to work to its full potential. More information appears below.

 

TerraServer Projection and Datum

If you intend to work with TerraServer images, you'll need to know the map projection and datum names if you intend to use the images in geospatial programs like Global Mapper or Map Maker Gratis. The projection and datum information places the image in the correct position in relation to the rest of the world. TerraServer uses the Universal Transverse Mercator (UTM) Projection and the NAD83 datum. When opening UTM data in a geospatial program, you might also be asked for a zone number.

So what is the Projection/Datum/Zone? Simply stated, a projection is a system used to display the curved surface of a spheroid (the Earth) on a flat map. The datum is a numbered grid placed on top of the map, used for designating geospatial locations. The zone is a segment of the map. UTM zones help to minimize distortions that result from the projection.

For a more precise explanation of map projections and datums, check the USGS site or the excellent article, "Where in the World are We?" from Australia.

UTM Zone numbers for the US are shown in the map above. A UTM Zone Map of the world is also available.

Manual TerraServer Downloads

When you are ready to save the TerraServer image you want, click "Download" from the top TerraServer menu line. TerraServer assembles the tiles that make up the image into one picture. You can then right-click the image and choose "Save As" to put it on your local drive as a "jpg" file. (You might need to manually add the "jpg" extension to the file name by using Windows Explorer*.) If you attempt to save the image before hitting "Download", you will only get a small tile of the picture that falls under the cursor. Use "Download" first.
To capture the image's projection and datum information, you must also save its "GIS World Coordinates".
The TerraServer download page gives you the chance to save the "GIS World Coordinates" as shown to the left. If you click that choice, text will appear in a separate window (see sample). Geospatial programs will use numbers in the file to correctly position the image relative to other geographic data you might overlay. Use the "File/Save As" function of your web browser to save the numbers as a "txt" file. Put it in the same place on your hard drive as image saved in above.
In order for Global Mapper and other GIS programs to use the TerraServer aerial photo and the geographic coordinates together, you need to use Windows Explorer to change the names of the two files. First, give the image a simple name and make sure it ends with a "jpg" extension. (Some browsers do not automatically give files an extension name, so you might need to do this manually.) Second, the text file must also be renamed. The part before the extension must be the same as the image file name. You must then remove the "txt" extension and replace it with "jpgw". (For example, you might have an image named "Delton.jpg" with its world file coordinates named "Delton.jpgw", both in the same folder.)

*Note: Changing file names and extensions can be difficult with Windows Explorer. You might want to get a free utility called PropertiesPlus, described in the Toolbox. To use it, you'd right-click a file's name, choose PropertiesPlus from the list that appears and then make the changes you want from the dialog box that opens.

Automatic Downloads with USAPhotoMaps

USAPhotoMaps is an exceptional tool developed and distributed by Doug Cox. It will automatically retrieve and display TerraServer aerial photos or topographic maps. It also connects to GPS units to display tracks and waypoints or real-time locations, as explained below. (Note: After downloading and unzipping USAPhotoMaps, you might want to make a shortcut to the program on your desktop or in a Start menu folder. The program code is lean - only about 240 KB zipped - leaving shortcut creation to the user.)
  • After starting USAPhotoMaps, you begin the process of image retrieval by choosing "File/New Map File". Provide an initial latitude/longitude for your project in the dialog box that opens:
  • The screen will first display a grid of gray, blank placeholders. Click on "File/Download Map Data/Fill Screen" (or tap "F" on the keyboard), and the tiles begin to fill in as shown here:

You can scroll the screen with the arrow keys on your keyboard or zoom out (Page Up key) and ask the program to again fill the screen. The program will connect with TerraServer and fill in the blank tiles until you have all the images you need.

How long does the download procedure take? If you want an image about 4.5 square miles square (about what you'd fit on a computer monitor at 1024x768 resolution when the zoom is set to four meters per pixel), USAPhotoMaps will retrieve approximately 1.6MB of data. 280 TerraServer tiles at 1 meter resolution are downloaded. With an Internet connection receiving data at about 30KB/second, it takes a little less than one minute to download the images. The process is smooth with a broadband connection, but some people may experience hang-ups if they are using a slow modem.

If you want to use the downloaded images in a geospatial program like Map Maker or ArcExplorer, you'll need to use the procedure described below to merge the tiles and generate a GIS World Coordinate file.

Merging Images with USAPhotoMaps-BigJPEG

The USAPhotoMaps download page includes a utility called BigJPEG. BigJPEG will assemble the image tiles downloaded by USAPhotoMaps into one georeferenced image. To operate properly, a copy of the BigJPEG program file must be moved to the directory where the image tiles are stored (generally the "USAPhotoMapsData" folder). Use Windows Explorer to place the file, and then double click BigJPEG.exe to start it.

As shown to the left, BigJPEG asks for the UTM coordinates for the northwest and southeast corners of the area to be assembled. You can get the necessary UTM coordinates in USAPhotoMaps by toggling the view between Lat/Lon and UTM as shown below. The UTM coordinates of the cursor position are displayed in the title bar. (Move the cursor to the top left and lower right corners of the area you want to assemble as a single image and write down the UTM values for use in BigJPEG.) You can also get UTM coordinates by using the the TatukGIS Coordinate Calculator.

 

Using USAPhotoMaps with your GPS Unit

If you have a GPS Unit, USAPhotoMaps can connect to it and display your waypoints and track directly on the TerraServer images. The program will work with any brand GPS receiver as long as it comes equipped with a serial cable for the PC.
In this example, USAPhotoMaps displays the track in blue and a waypoint as a green dot. (Click the image for additional information.) The program will add text labels where you choose.

You can use USAPhotoMaps to manage your GPS track and waypoint records, retrieving past activities to reload into your GPS device.

You can create new waypoints on a TerraServer image (either on a photo or a USGS topographic map) by left-clicking on locations with your mouse. The program asks for a waypoint name, which you provide. You can send the newly created positions to your GPS unit so you can find those locations in the field.

If you travel with a GPS unit and laptop PC, USAPhotoMaps can even be used as a live display to show your location on a photo as you move. If you plan to use the program that way, download your background aerial photos in advance.

Merging Images with Global Mapper

If you need a number of TerraServer download images to cover the area you want to map (say you want a detailed aerial photo covering four square miles at 1-meter resolution), how would you put the photos together as one image? Global Mapper can do it in a snap. Here's how:

1. Before you start downloading multiple images from TerraServer, create a directory that would hold them. For example, you might make a directory named "Delton" for your Lake Delton aerial photos.

2. Download each photo you want (click on the TerraServer compass rose to move to different images to download). Name the images consecutively (e.g., download1, download2, etc.) You must also save the corresponding "GIS World Coordinates" as download1.txt, download2.txt, etc.

3. After you have all the pictures you want, find the image and text files with Windows Explorer. Select all the image files, right click them, and add a "jpg" extension to each. (This can be done as a batch with PropertiesPlus.) Also, select all the text files and change their extension to "jpgw".

4. You can now start Global Mapper and open each of the photos into the current view. Global Mapper will put them all together as a single image. If you have the registered version of Global Mapper, you can save the whole group as a single GEOTIFF file. If not, you can still get a screen capture of the assembled images. (Before doing a screen capture, you might want to ramp up the resolution of your monitor as far as it will go. The more pixels you capture, the better the image.)

 

GEOTIFF, MrSID, ECW or other Digital Image Files

You might have access to DOPs or DRGs (usually in some sort of compressed format like MrSID or ECW) from commercial vendors or from natural resource agencies. See Basic Map Data for sources. Programs like Global Mapper, ArcExplorer 2, MrSID GeoViewer, MrSID Viewer, ERDAS ViewFinder and others can be used to view those images.

Geospatial images might cover a much larger area than you want to map. The programs mentioned above allow you to clip out a portion of a large file and save it as a conventional image file. Some of the programs will save the GIS World Coordinates of a clipped image. The registered version of Global Mapper is probably the easiest tool for saving a clipped image along with the world file coordinates. The free LizardTech MrSID utilities will do a nice job for files in the MrSID format.

You can also get clip images from programs like Global Mapper and ArcExplorer through screen captures. You can use those programs to create images with multiple DLG layers (section lines and streams are often among the most useful to add over the tops of aerial photos). When you have a view you like, take a screen shot. (It might help to ramp up your monitor resolution before doing so.) If you wish, you can geo-reference the captured image.

As an example, Figure 1 is a screen capture from Global Mapper showing a Digital Elevation Model (DEM) and Digital Line Graph (DLG) transportation data for Devil's Lake State Park in southern Wisconsin. (Hill shading in Global Mapper has been turned off.) After capturing the view but before leaving Global Mapper, the cursor was moved to both the upper left and lower right corners, and the latitude/longitude of each location was noted for the next step.

Figure 1
As an example, Figure 1 is a screen capture from Global Mapper showing a Digital Elevation Model (DEM) and Digital Line Graph (DLG) transportation data for Devil's Lake State Park in southern Wisconsin. (Hill shading in Global Mapper has been turned off.) After capturing the view but before leaving Global Mapper, the cursor was moved to both the upper left and lower right corners, and the latitude/longitude of each location was noted for the next step.
Figure 2
In Figure 2, the screen capture is used as an overlay in 3DEM to create an oblique 3D view of the park. The view can be rotated to any angle. The latitude/longitude of the two corners (saved as explained above) orients the overlay to the underlying elevation model.

 

Geo-Referencing Aerial Photos that are Scanned for Base Maps

If you have an aerial photo print or paper map (perhaps more recent than TerraServer images or for an area where you don't have digital coverage), you can use a flatbed scanner to convert the image to digital format. Some of the programs in the Toolbox like 3DEM, OziExplorer and Map Maker have fairly simple methods for registering a scanned image for internal use of the programs. If you want to use a scan with other geospatial data, however, you will need to create a GIS World Coordinate file for it. The following tutorial explains the contents of a world coordinate file and how to create one.

Important!
If you expect to routinely use scanned aerial photographs for base maps in geospatial mapping programs, the images should be corrected for distortions so they reflect true ground positions. The procedure is called rectification. Click here to learn more.

 

Approximating a UTM World Coordinate File for a Scanned Image

Professional GIS programs like ArcView have extensions for image registration, but you might not have access to such a program. Following is a relatively simple procedure to geo-reference a scanned photo with free resources available on the Internet. It is not exact and cannot be used for a scan that covers more than a few square miles.

  1. On the photo to be scanned, mark two points that are a known distance apart. If nothing else, mark two points a mile apart based on the photo scale. If you have a thin ruler or printed scale, you could instead lay that on the photo and include it in the scan.
  2. Scan the photo (or part of it) at a resolution that captures enough information for your purpose without creating an image file that will overwhelm your computer's memory.
  3. In your photo-editing program, make any adjustments you want for contrast/brightness/sharpness. (You might also need to rotate the image by the magnetic declination of your area so that it is aligned to the grid north of any other projection/datum for data you intend to use with the scanned image. This might be a trial and error process.)
  4. In the photo-editing program, you will need to crop the picture so the upper left (NW) corner falls on a landmark (e.g., a road intersection, field corner, etc.). You need to be able to see the same landmark on other geo-referenced data (such as a TerraServer image, DRG or DLG source that can be viewed in a program like Global Mapper or USAPhotoMaps that gives UTM coordinates). Save the cropped image as a "jpg" file with minimal compression.
  5. Next, you need to create a GIS World Coordinate file for the scanned image. The coordinates will be stored in a text file that has the same name as the image file, except it must end with a "jpgw" extension. The text file would contain numbers similar to the right column in the example below:
A. X-scale (meters per pixel in the X direction)
1.282090
B. Rotation in X direction (assumed = 0)
0.000000
C. Rotation in Y direction (assumed = 0)
0.000000
D. Negative of Y-scale (meters per pixel in the Y direction)
-1.282090
E. UTM Easting Coordinate of the center of the upper left pixel of the scanned image
730691.000000
F. UTM Northing Coordinate of the center of the upper left pixel of the scanned image
4797771.000000

That is, the "jpgw" text file for the example would look like this:

1.282090
0.000000
0.000000
-1.282090
730691.000000
4797771.000000

The trick is to get the values for A, D, E and F.

As implied in Step 4, E and F can be found with programs like Global Mapper or USAPhotoMaps that provide a readout of UTM coordinates as you move the cursor across the screen. While viewing geospatial data with such a program, observe the UTM coordinates of the landmark used for the NW corner of the scanned image. Enter the values for E and F in your table. Here's a screenshot from USAPhotoMaps showing how to get UTM coordinates:

There is a formula to estimate the values of A and D based on the dpi of your scan and the scale of the parent photo you scanned. I've had better success, though, by importing the scanned photo into Map Maker Gratis and getting the scale factor from that program. Gratis has a bitmap utility to calibrate a scan.

Use the "single point + known distance" procedure on the photo marks (or on the ruler you included in the scan) made in Step 1:

a) Open the scanned photo with the Map Maker menu choice shown above

b) Pan the photo to find the first marked reference point from Step 1

c) Right-click the first point to start the calibration

d) Place a cross-hair on the first point and click it

e) Pan to the second reference point from Step 1; point and click it

f) Enter the distance in meters between the two points in the dialog box that opens (if the two points you chose are exactly one mile apart, enter 1,609.344 meters).

Map Maker Gratis then gives you an option to save a separate world text file for the image or to use a proprietary registration method. For this procedure, choose the world file option. Map Maker creates a text file with a "jpw" (not "jpgw') extension.

Open the "jpw" text file that Gratis makes with Notepad or other text editor and observe the values it uses for the A and D pixel scale factor. Copy those values into your own "jpgw" text file table that has the UTM coordinates for the origin (E and F).

Of course, if you intend only to use the scanned photo in Map Maker, the registration file it creates will be adequate. With it, Map Maker will calculate reasonably accurate areas and distances on the scanned photo. The world file with the UTM coordinates, however, can also be used with other geospatial data (but remember to enter the correct UTM Zone number if needed).

Use these procedures at your own risk. They will only approximate a world file registration for a scanned image. Precision image registration is much more complex, requiring rectification of the image.

 

*** Back to 2D Maps for Resource Management ***

 
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Last Updated 2/2/2012 6:37:19 PM