Preserving the Past, Informing the Future
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A "casual user of these images is wise to spend a few minutes learning to distinguish a small cloud shadow from a small lake, an air force base from a civilian airport, or a stand of young timber from a grassy hillside. … [U]sers are likely to examine the photographs and USGS topographic maps together, perhaps alternating between the two to connect place names and road numbers with unidentified features in the photographs." From Interpreting Aerial Photographs from the United States Geological Survey, Digital Backyard and the National Aerial Photography Program.
Washington (State) Dept. of Ecology Shoreline Aerial Photographs The site has a nice drawing to illustrate how to use the layers to find a photo.
Geo-referencing is a painstaking process to determine the latitude and longitude of four or five points on a map, aerial photograph or aerial mosaic panel. With that information and the right software, every point on the map or photograph is associated with its own latitude and longitude. Because they are geo-referenced, users with sophisticated software can merge and layer information from different geo-referenced maps and databases to make new maps. The 1934 Connecticut Aerial Photograph Mosaic was created from 239 images that were geo-referenced and combined to make one statewide photograph. See the Project Report.
The U.S. Geological Survey drew a grid on a map of the United States. Each box on the grid is a quadrangle. Map makers refers to their maps as quadrangles when the area represented on the map is the same as the area within one of the standardized boxes on the USGS map. Quadrangle maps are also called quad maps.
Index Sheets show the location of aerial photographs relative to each other and give an indication of what part of the state is shown on the photographs. The Connecticut State Library has aerial surveys from 1934, 1951 and every five years starting with 1965. Each year has a different set of index sheets, and the method for using each set is unique. Some are drawn over U.S. Geological Survey topographic maps. On these index sheets, a box shows the outline of each photograph or a dot shows the center point of each photograph. Other index sheets are a composite, or mosaic, of all the photographs showing how they relate to each other. Refer to the Research Guide to Aerial Photographs at the Connecticut State Library for instructions specific to each survey.
U.S. Geological Survey, Frequently Asked Questions: What is a topographic map?