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.
Other Digital Image Files
Geo-Referencing Aerial Photos that
are Scanned for Base Maps
- TerraServer Aerial Photo and Map
TerraServer Aerial Photo and DRG
||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.
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"
|To capture the image's projection and datum information,
you must also save its "GIS World Coordinates".
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.)
||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.
*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
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
Using USAPhotoMaps with your GPS
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
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
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
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
GEOTIFF, MrSID, ECW or other Digital
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.
|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.
|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.
|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
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.
- 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.
- 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.
- 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.)
- 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.
- 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:
||X-scale (meters per pixel in the X direction)
||Rotation in X direction (assumed = 0)
||Rotation in Y direction (assumed = 0)
||Negative of Y-scale (meters per pixel in the Y
||UTM Easting Coordinate of the center of the upper
left pixel of the scanned image
||UTM Northing Coordinate of the center of the upper
left pixel of the scanned image
That is, the "jpgw" text file for the example would look
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
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
b) Pan the photo to find the first marked reference point from
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
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
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.