Topographic maps are characterized by the use of contour lines to show land relief or terrain features. Contours represent lines of equal elevation and spacing between lines is a fixed value. Closer contour lines describe steeper terrain features. Commonly used 7.5 minute topographic or topo maps are printed on a 1:24,000 scale, where one inch represents 2,000 feet on the ground. Many times these topo maps are referred to as 7.5 minute quadrangles or quads.
Quadrangle maps are labeled by the geographical area they represent and provide a suggestion to the viewer of the area’s size. A large format topographic map typically used by researchers measures an area of two degrees of longitude by one degree of latitude and is called a two degree sheet. It is printed at a scale of 1:250,000. A map of this size does not provide the detail available in smaller scale maps. A two degree sheet can be subdivided into four smaller quads covering one degree of longitude and 1/2 degree of latitude at a scale of 1:100,000.
Unsurprisingly these are known as one degree sheets and they can further be divided into eight fifteen minute quadrangles. Finally, a fifteen minute quad can be divided by four into the smallest map published by the USGS, the 7.5 minute quadrangle. If you lost track of all the division, there are 128 7.5 minute quads in a two degree sheet.
The USGS took on the task of mapping the United States in 1879 and remains the lead civilian mapping agency in the country. Technology has marched on and historic quad maps are being replaced with new US Topo quadrangles that offer many improvements over their older cousins.
Concerning the updated maps. USGS Project Manager Bob Davis says, “The newly redesigned US Topo maps are visually appealing, especially with the addition of the shaded relief layer. The addition of shaded relief and other design components demonstrate our commitment to improving the product to meet our users’ needs.”
US Topo Map Enhancements:
- A crisper and cleaner design that improves online and printed readability while retaining the look and feel of traditional USGS topographic maps
- A new functional road classification scheme has been applied to the maps
- A slight screening or transparency has been applied to some features to enhance visibility through multiple competing layers
- Fonts are being used that support diacritical marks to add sound values to letters
- A map symbol legend that is presented in a separate PDF file
- Metadata formatted to support multiple browsers
- Shaded relief layers for enhanced view of the terrain
US Topo quads are freely available for download at the USGS. The digital quads include geospatial extensions of geographic data layers for things like orthoimagery, names, hydrography and contours that are found in the National Map dataset. The maps can be viewed with any device that supports Adobe Reader or another PDF viewing program.
Alaska’s maps are scheduled for updates and publication later in 2013. With revisions scheduled every three years, topo maps for the Great Land should be more accurate and accessible going into the future.
Copyright © 2013 by Alan Sorum