Why am I seeing a strange looking topo map in a swamp / marshy area?

A map image that you might come across in our topographic map collection is an "orthophoto".

In some locations, usually swampy or coastal areas, the U.S. Geological Survey did not create a regular USGS colored map. Instead, in order to better display the nature of the terrain, they used an aerial photo of the area and then overlaid contour lines and other features. This creates a map much different in appearance from the normal USGS maps.

In any case, these maps represent the last classic USGS quadrangle produced in that area, so it is the only map image we have.

The newer US Topo maps (available by quad sheet only) do not utilize the orthophoto base map.

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