About the Georeferenced Map Image Download Tool
This purpose of this tool is to provide users with a downloaded, georeferenced image of the map (with same displayed extent and features) that is displayed in the map viewer.
2 image sizes are available for download:
- 96 dpi image - this is an exact copy of the displayed image) and is best used for printing at the size that is shown on the screen.
- 300 dpi image - this is a much larger sized image than the 96 dpi, but has a much better resolution. It is best for use in GIS software.
A datum and projection for the georeferenced image can also be selected from the menu. The default datum and projection is North American Datum 1983, Kentucky Single Zone.
When an image is downloaded, a .ZIP file is produced containing the following 3 files:
- a .tif file: this is the image file in TIFF format.
- a .tfw file: this is a "world file" which provides georeference information for the TIFF file. IMPORTANT: the .tfw file must have the same name as the .tif file in order for the image
to georeference according to the data provided in this file.
- a metadata (.txt) file containing the coordinate, extent, and resolution information for the downloaded image. This is a text file which can be opened in any text editor (e.g. Notepad).
For an orthogonally rectified image, the world file (.tfw) reads as
follows:
Line 1: Cellsize in the image x direction.
Line 2: Zero
Line 3: Zero
Line 4: Cellsize in the image y direction (negative of Line 1, see note
below).
Line 5: x coordinate (in the selected datum and projection) for the center of the upper left pixel.
Line 6: y coordinate (in the selected datum and projection) for the center of the upper left pixel.
Note:
Line 3 (cellsize in the image y direction) is based on the image y axis
being positive downward while the standard Euclidian y axis is positive
upward. This means that the Line 3 value is always the negative of Line
1 for an orthogonally rectified geoimage with square pixel dimensions.
DO NOT ALTER THE INFORMATION IN THE .tfw FILE OR THE .tif FILE WILL NOT BE GEOREFERENCED PROPERLY.
The downloaded files are provided by the Kentucky Geological Survey. Questions? Comments? Call 859-257-5500 or email Doug Curl - doug@uky.edu