Automatic TerraServer Downloads with USAPhotoMaps

USAPhotoMaps is an exceptional, free tool developed and distributed by Doug Cox. Use it to automatically retrieve and display TerraServer aerial photos or topographic maps. The program also connects to GPS receivers to display tracks and waypoints or real-time locations (below). A companion utility, BigJPEG, merges TerraServer image tiles into geo-referenced files that can be used in other spatial mapping programs.

USAPhotoMaps comes packaged with a simple installer that unzips the program and places a shortcut (below) on your desktop.

After starting USAPhotoMaps (double-click the shortcut), you begin the process of image retrieval by choosing "File>New Map File". Provide an initial latitude/longitude or UTM coordinates for your project in the box that opens. For the Lat/Lon, you can use just decimal degrees or enter degrees, minutes and seconds if your location is in that format. See Digital Grove Links for sites that will quickly provide the Lat/Lon of a location you are interested in.

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 (tap the "F" key). 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 1024×768 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 fGIS, Map Maker or Global Mapper, 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.exe program and the file "ijl15.dll" (a USAPhotoMaps application extension found in the USAPhotoMaps folder) must be copied to the directory where the image tiles are stored. Aerial photo image tiles are stored in the "BigPhotoMapsData " folder, whereas topographic map image tiles are saved in the "USAPhotoMapsData" folder. You might want to copy BigJPEG and its dll to both data directories. You could create desktop shortcuts to the BigJPEG program or start the program file by double-clicking it from within Windows Explorer.

Since BigJPEG is simple code, it requires some user interaction in the following procedure:

From the USAPhotoMaps "File" menu, click "Boundaries for BigJPEG".
Drag a bounding box (depress the left mouse button and pull the mouse down and to the right) over the image tiles you want to join. Release the mouse button. The program will then write a small text file that records the corners of the area you selected. The text file is named based on the UTM easting, northing and zone number of the box’s top left corner. You’ll see a notice like the following when that step is finished:

Start BigJPEG from either the photo or topo data directories. It will search for text files with naming conventions like that shown above. Cycle through the choices by tapping the cancel key until you see the file you want listed:

Click OK, and BigJPEG will save an image file (ending in "jpg") and a world coordinate file (ending in "jgw") in the USAPhotoMapsData folder. You might want to move those two files to a project folder. If you rename the files, make sure the first part of each file name is identical (such as Camp_Douglas.jpg and Camp_Douglas.jgw). That way, mapping programs can find the world coordinate file associated with the image.

The world coordinate file from the preceding process would look something like the following if you were to open it with a text editor:

Click here if you want to know what each line means. Since the names of the map projection and datum are not part of the world coordinate file, many mapping programs (including Global Mapper) will need you to provide that information. TerraServer aerial photos and topographic map images use the Universal Transverse Mercator (UTM) projection and the NAD83 datum. You’ll also need to provide the UTM zone number. Using the world coordinate file and the name of the projection/datum/zone, mapping programs can correctly position the image in relation to other geographic data.

Using USAPhotoMaps with your GPS Unit

If you have a GPS Unit, USAPhotoMaps can connect to it and display your waypoints and tracks directly on TerraServer images. The program will work with any brand GPS receiver as long as it comes equipped with a serial cable for the PC and supports either a Garmin or NMEA protocol.

You will need to set up USAPhotoMaps so it can find where your GPS unit is connected and so it knows what transfer rate to use. Here are the settings for my Garmin unit (yours will probably use Comm 1 or Comm 2):

Select GPS>Track>Receive, and the track points will be saved as a file and displayed as a poly-line overlay on the photo as shown below. Waypoints can also be received and displayed in a similar manner.

Another very useful feature of USAPhotoMaps is its ability to send waypoints to your GPS unit. Say you want to explore a parcel of woodland, and you’d like to know when you are getting close to the corners of the property when you are out walking around…

You can pick and name the property corners (or other features) on an aerial photo:	…and use USAPhotoMaps to send them as waypoints to your GPS:
USAPhotoMaps Screen Capture	Garmin GPSMAP 76 Screen Showing Waypoints Transferred by USAPhotoMaps

As you navigate with the GPS unit i
n the field, you will be able to use the corners that you marked and sent as reference points. If you are also using the optional "Convert SDTS Data" terrain elevation utility (found at JDMCox Software), the GPS points that you send to your GPS unit with USAPhotoMaps will include USGS DEM height data.

With the Convert SDTS Data tool installed, elevation contours will be displayed as an overlay. Simply download zipped 30-meter SDTS DEM files to the USAPhotoMaps folder and run the Convert SDTS utility. It will unzip the files and adapt them for use by USAPhotoMaps. When elevation data is present, it is displayed as contour lines and as an elevation reading in the title bar for the position of the cursor. Waypoints sent to a GPS unit will also carry the elevation values.

If you can’t pick out the points you want to transfer from the aerial photo, you can switch the view to the topographic map option:

The topographic map view provides section lines and other reference marks that might be clearer than on the aerial photos. The resolution of the topo maps is limited, however, to four meters per pixel (as compared to one meter per pixel maximum for the aerial photos).

Real-Time Location Display

If you travel with a GPS unit and laptop PC, USAPhotoMaps can display your real-time track on a photo or topo map as you move. Simply connect the GPS unit to your laptop PC via a serial cable, start USAPhotoMaps, and select "GPS>Show Location". You might also want 12-volt cigarette lighter adapters for your PC and GPS unit so they can be connected to your car or truck’s power supply or external battery.

If you plan to use the program in real-time, download or transfer your background image tiles to the laptop or a CD in advance. The "USAPhotoMaps.txt" file that comes with the program explains the procedure for saving image tiles to a CD. Take care when you are in a moving vehicle to watch the road, not your laptop screen.

Digital Grove offers simple, practical information about digital cartography, GPS and Geographic Information Systems.

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