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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 Sattelite 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 pizel 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 arer usually good enough to tell developed land from agricultural or foerested land. Even different types of development can be discerned, if you know what you are doing. FOr more information see Notes on Multispectral Images and Multispectral Image Classification Tutorial.

    Broad-scale Georeferenced Aerials for Anywhere, for a Cost

    • Digital Orthophoto Quads: The US Geological Survey has been saving high-resoultion 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 isn't necessary. If you need to georeference an image, see the notes toward the end of this document.

    • Amazing Boston Maps: the The BRA Boston Atlas. is an amazing tool that brings together scores of historic map, and lets you look at places over time.
    • 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 spendid 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!
    • Scanned Hostoric Sanborn Maps The Sanborn Mapping Company has been making very detailed maps of the built environment of the US sincr the 1860s. Harvard users are very lucky to be able to access Huge amounts of these maps without leaving your seat: Click Here to Access the Scanned Sanborn Collection Requires a Harvard PIN.
    • 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.
    • High-Res Photos from Google Earth Pro Probably the best source of high-resolution aerial photography for many places. Use the Google-Earth-Pro installation at your local library (for example, at Harvard: the Fracnes Loeb Library at the GSD or the Map Collection.) To get images, zoom to your area of interest and choose File->Save->Image Note that you can georeference Google Earth Images with the bootstrap technique discussed at the end of The GIS Manual Page on Georeferencing Images

      Harvard's Map Collection

      After you have looked around on the web, you may want to stop in to Harvard's Map Collection where they have tens of thousands of maps that can be scanned for a nominal fee.