Geographic Data Resources

Finding Geographic Images

Finding Geographic Images

This page discusses sources and techniques for dealing with geographic images, including current Aerial photography and scanned maps, as well as historic maps. We will look at some sources for images, some of which are georeferenced and ready to overlay with GIS data, and there are many sites with old maps that aren't georeferenced.

References

Georeferenced Images

A georeferenced image contains information, either within itself, or in a supplementary file (a world file), that explains to a GIS system, how to align the image with other data. Formats that support built-in georeferncing include GeoTiff, jp2, and MrSid. Other images may carry georeferencing information in a companion file (known in ArcGIS as a world file, which is normally a small text file with the same name and suffix of the image file, except fopr the last character, which is replaced by a 'w'. (e.g. myimage.tif / myimage.tfw) Any image can be georeferenced, see Georeferncing Images.. If you have an image with a world file that still does not line up with other layers, see The Projections Tutorial.

There are several sources of georeferenced images available on the web. Some are free and some cost money:

Free Georeferenced Images

  • Free Satellite Images (Time Series) for anywhere in the world! The Maryland Global Land Cover Facility Has tons of satellite images from NASA, covering virtually the whole world and at many tifferent epochs in time. Note that these images very LOW-Res (ordinarily each pixel is 30 m.) The images are multispectral which means that they have information about parts of the spectrum that we can't see. Yet they are usually good enough to tell developed land from agricultural or forested land. Even different types of development can be discerned, if you know what you are doing.

    Broad-scale Georeferenced Aerials for Anywhere, for a Cost

    • Digital Orthophoto Quads: The US Geological Survey has been saving high-resolution aerial photos of the entire country since around 1993. These are available for a fairly inexpensive fee. This is a good option if you can wait a few days, it will save you from having to tile together lots of relatively small images. See Earth Explorer

    Non-Georeferenced Maps and Aerials From the Web

    There are several sites that make scanned maps and digital aerial photos available for free These may not be ready for insertion into your GIS, but for many purposes this is not necessary. If you need to georeference an image, see the notes toward the end of this document.

    • Norman Leventhal Map Center at the Boston Public Library The LMC has scanned thousands of amazing maps of Boston from 1630 through to the present!.
    • Historic Maps of Lots of Places: The David Rumsey Collection. Mr. Rumsey is evidently a map fanatic, a scholar and philanthropist. He has scanned tens of thousands of antuiqe maps and made them available -- with very good metadata! -- for free. Hats off to you Mr. Rumsey!!!
    • Historic Aerial Photos of US Places Back to 1955 from the USGS Aerial Products from USGS
    • The Library of Congress American Meomory Project: On-Line Map Collection features many splendid maps especially bird's-eye-views of American cities from the 19th century. The Maps Category of American Memory Project
    • The University of Texas Casteneda Library: On-Line Map Collection has tons of maps from all over the world, and a handy list of other on-line map sites.
    • Historic USGS MAps for New England and New York: University of New Hampshire Library from the begining of the century!
    • National Oceanographic and Atmospheric Administration Historic navigational charts Not for navigational purposes, but these old maps provide a lot of valuable information about the history of our ports and coastline.