Topo maps are handy when used with recent aerial photos.
A land boundary is anything that affects the landscape. Examples include glaciers, ice fields, canyons, shifting dunes or cliffs. Land boundaries are represented on topographical (topo) maps like those published and maintained by the United States Geological Survey (USGS), a division of the US Department of the Interior. Aerial photos are those taken from aircraft, satellite or hot air ballon and are used to coincide with topo maps. Aerial photos are one piece of data USGS cartographers use to update topo maps. Aerial photos, when used with current topo maps, are handy references for finding and interpreting land boundaries.
Instructions
1. Spread out the USGS topo map of the area you are studying. Topo maps are available free in electronic form and may be printed out. Paper versions are available through the USGS or most outdoor outfitters. Locate the specific areas on the map you are trying to decipher. If you are unsure, use GPS datum coordinates or a set of latitude and longitude measurements.
2. Set out the aerial photographs that coincide with the region of the map you wish to study. Look on the bottom right hand corner of the aerial photos to find the scale to which the source camera was set; another measurement to look for is the height from which the photos were taken. Use this data to help determine the size of the land barriers on the topo map. For example, if you know the aerial photos were taken 10,000 feet AGL (Above Ground Level), you will use this data to interpolate heights on the topo map. You will also use this information to determine the striation lines on the map that indicate altitudes.
3. Look for canyons, mesas, mountain peaks, rivers, dry river beds and fault lines on the aerial photos. For example, if you look at aerial photos of the Glen Parks Highway in Alaska, you will find evidence of the Denali Fault. The Denali Fault is a natural land barrier that runs north to south, sometimes parallel to the highway. Now find the same land barrier on the topo map and familiarize yourself with the icons and markings used on the map; use this for future interpretations of similar land barriers on maps.
4. Determine if land barriers have moved or changed since the last updates to the USGS topo maps. Look on the lower left hand corner of the USGS topo and find a date indicating when the maps were last updated. Land barriers change over time due to plate tectonics, earthquakes, erosion and climate changes. Examples include glaciated regions. If you look at the quadrangle for the Dyea Valley of southeast Alaska from over two decades ago, you see a land barrier on Mt. Harding indicating a long valley glacier. Today, that glacier is nearly gone, yet the maps have not been updated to indicate this land barrier change. By using aerial photos of the region in conjunction with the topo maps, land barriers are interpreted for the map datedness.
Tags: land barrier, aerial photos, topo maps, USGS topo, Aerial photos, aerial photos
