A wealth of downloadable free and fee maps exist for use with ArcView and ArcExplorer

View Examples of the Various Types of Downloadable Maps - USGS

1. Satellite images - DOQs (Digital Orthoquadrangle Imagery) hi-res usually b&w

• Because the USGS provides these as GeoTIFFs (georeferenced TIFFs), they may be viewed with image rendering programs, web browsers, image processing systems, and geographic information systems to easily read and display DOQ images. This means that you can open DOQs in any software that reads TIFFs and you can use image editing software to combine images, merge them to form a mosaic, and superimpose thematic data using the DOQ for geographic reference.
• The USGS is preparing some as and sometimes Spatial Data Transfer Standard (SDTS). To read this file format you must have ArcGIS software that can handle SDTS and integrate all the georeferenced metadata.

2. Topographic maps (terrain relief with contours) - DRGs (Digital Raster Graphics)

3. Plane - very accurate maps

4. Aerial photos - (not maps) distorted by camera tilt, topography, and inconsistent scale

Differences between aerial photographs and orthophotos

1. A conventional perspective aerial photograph contains image displacements caused by the tilting of the camera and terrain relief (topography). It does not have a uniform scale. You cannot measure distances on an aerial photograph like you can on a map. It is not a map.
2. The effects of tilt and relief are removed from the aerial photograph by the rectification process to create an orthophoto.
3. An orthophoto is a uniform-scale photograph. It is a photographic map.
4. Since an orthophoto has a uniform scale, it is possible to measure directly on it like other maps.
5. An orthophoto is map onto which other map information may be overlaid.
   
   -- Courtesy USGS Western Geographic Science Center

5. DEMs - Digital Elevation Models

DEMs may not open in ArcView GIS - USGS DEM data updated since (2001) may not import correctly using the import utility that comes with ArcView GIS (or other programs). Unless the software you are using supports the new SDTS header format you will have to run the SDTS2DEM utility on the data - See http://www.gisdatadepot.com/dem/sdts2dem.html

6. DLGs - Digital Line Graphs

The USGS now offers Digital Orthoquadrangle (DOQ) in a format called GeoTIFF, a georeferenced version of the popular Tagged-Image File Format ( TIFF ). This allows users with image rendering programs, web browsers, image processing systems, and geographic information systems to easily read and display DOQ images. If you have a program that can display TIFF, you can read DOQ. While the DOQ is encoded as a TIFF image, there is more that you need to know about the USGS product.

First we should describe GeoTIFF . If you have a program that understands GeoTIFF, the geographic reference of the image will be read in along with the image. This is an important kind of "metadata" for people who intend to use the image in conjunction with other geospatial data. This georeferencing information can be manually input to most programs that need it, but differences in reference models and input syntax can make this process time consuming and prone to error.

The Spatial Data Transfer Standard (SDTS) allows a GeoTIFF image to be included with other files that contain cartographic metadata. The USGS is working on implementing this for DOQs. If you are fortunate enough to have software that understands GeoTIFF and also understands Spatial Data Transfer Standard (SDTS) , you will get the full benefit of this DOQ product.

File Formats

JPEG2000 or MrSid - Free Map Viewers (JPEG2000 are smaller and more manageable)

Projections are representations of the earth's surface on a map. While more than 400 projection systems exist, none can perfectly represent direction, distance and area.

Latitude and longitude are a geographic coordinate system. Universal Transverse Mercator (UTM) projections are an example of a projected coordinate system, where each point on the earth can be expressed with x,y coordinates.

Labins - Land Boundary Information Systems - maps for Florida

FDOT (Florida Department of Transportation) files are now available in what is referred to as personal geodatabases which include the shapefiles and feature datasets (base map coverage). The latter is available as easy to use layer files (.lyr).
Note: These data files cannot be read by ArcGIS 3 or ArcInfo, but can be read by ESRI's free utility called ArcExplorer downloaded from:  http://www.esri.com/software/arcexplorer/index.html

To use scanned maps, aerial photographs and satellite images (raster data) with other spatial
data, it is usually necessary to georeference them to a map coordinate system, also known as rectification. Ground control points are taken from a raster image and a mathematical algorithm warps them to the map so that the images fits together. The coordinates of points are taken from a paper map and may be entered into ArcGIS to obtain a correctly registered alignment. This transformation of raster data allows it to be
viewed, queried, and analyzed with other geographic data.

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This page was last modified 1/28/07 .  Comments and suggestions to: karolyne@empoweringtech.net