2D Maps - TerraServerUSA Aerial Photo and Map Images
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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. TerraServerUSA Aerial Photo and Map Images 3on this page
  2. Other Digital Image Files
  3. Geo-Referencing Aerial Photos that are Scanned for Base Maps

 

TerraServerUSA Aerial Photo and DRG Images

As described in Basic Map Data, TerraServerUSA is an excellent source of free USGS 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. TerraServer 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.

After you've zoomed into a TerraServerUSA image that you'd like to use in a mapping program like fGIS or Global Mapper, you'll need to save both the view (a "jpg" image file) and associated world coordinate file (a small text file ending in "jgw") that tells a mapping program the image's position and scale.

To save the image, click the download button in the top TerraServerUSA menu bar. TerraServer then assembles the image tiles into a view that you can save to your computer by right-clicking it and giving it a file name. Make sure the saved file has a "jpg" (not "jpeg") extension.

The download page also has an option on the far left to save the World Coordinate File: Right-Click the link and choose to "Save Target As", giving the target the same name as the view, except use a "jgw" extension. Both the image file and the "jgw" text file must be saved in the same folder. If saved correctly, mapping programs like fGIS will open both the jpg and jgw files simultaneously.

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. See Digital Grove's instructions for using USAPhotoMaps.

 

TerraServerUSA Projection and Datum

If you intend to work with TerraServerUSA images, you may need to know the map projection and datum names. The projection and datum information places the image in the correct position in relation to the rest of the world. TerraServerUSA 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 mathematical model of the earth's shape, represented by a numbered grid used for designating geospatial locations. Various datums, or coordinate systems, are designed to take into account imperfections in the earth's lumpy, irregular shape due to gravity, plate tectonics and other forces. As a result, projections can employ many local datums, each optimized for a particular geographic area. A zone in the UTM system is a segmentation of the projection used to minimize distortions.

The UTM (Universal Transverse Mercator) coordinate system used by USGS for TerraServerUSA images is a method for minimizing distortions when representing a globe on a flat surface. 

Here's how it works:

Most people are familiar with the Mercator projection (below), a simple method of flattening a globe. Greenland, Antarctica and other areas near the poles are distorted, though, making them look huge. The part of a Mercator coordinate system with the least distortion is a belt along the equator.

To understand a Transverse Mercator (TM) projection, imagine tipping the world coordinate system sideways (transverse). The axis of such a map would go through a couple opposing places on the equator, and the undistorted strip (like that along the equator, above) would run north-south. Cartographers using a TM projection shift that undistorted strip around the globe in what they call zones. 

In a Universal Transverse Mercator projection, each of the relatively distortion-free zones is 6 degrees wide. Since the earth is 360 degrees around, the UTM coordinate system has 60 zones world-wide.  As shown in the following map from USGS, the continental US falls between UTM zones 10 and 19.

(Click here for a UTM Zone Map of the world.)

So where did the term "Universal" in UTM come from? The 60 zones and standard coordinate numbering system used in UTM were established by NATO in the 1950's as a mapping system for "universal" military use.

To learn more about the UTM coordinate system, check the University of Colorado's web site.

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