GEO 4120c Workshop

Digital Measurements

30 March 2009

Introduction

Basic photogrammetry is an important part of air photo interpretation. The sizes of objects on the ground are one of the most important characteristics for their identification, and many applications of aerial photography include measurements of the length and area of various objects. Sequential aerial photography is still one of the most important methods for studying how land has changed over time, but measurements must still be made. Digital aerial photography has reduced the need for manual measurement and has other advantages, but has its own constraints as well. Some of the issues are that aerial photo interpreters must learn basic GIS or Remote Sensing software programs for displaying and analyzing images; measurements are subject to errors of digitization, image spatial resolution, resampling, and over-specifying precision. This workshop will take you through the input, reprojection, and measuring differences between two different times for an area to the NW of Gainesville.

Sources of Ground Measurements for this Workshop - LABINS Digital Orthophoto Quads for USGS Topographic Quadrangle 4720: Alachua, Florida

· Florida Land Boundary Information System (LABINS) - gateway to DOQ of Florida

Equipment for this exercise:

ArcMap GIS Software
             8 Digital Orthophoto Quads from LABINS (in G:\share\GIS 4120c Aerial Photo Interpretation\Air_Photo_Example_Data\Digital Aerial Photography Images\); 2 from 

Exercise

1. Copy both folders G:\share\GIS 4120c Aerial Photo Interpretation\Air_Photo_Example_Data\Digital Aerial Photography Images\1995_alachua_DOQQ and G:\share\GIS 4120c Aerial Photo Interpretation\Air_Photo_Example_Data\Digital Aerial Photography Images\2004_alachua_DOQQ to your folder on the G: drive. There are four images in each folder.
2. Add all eight DOQs to the Map Window in ArcMap. These are aerial orthophotographs of the Alachua topo quad for 1995 and 2004.
3. Read the metadata on the LABINS web site for each set of images to determine which datums are used.
4. Select the datum of one of the sets of images, then reproject the other set to the same datum. If there are no units specified, then create units for the data layer, then reproject.
5. Start a new project in ArcMap
5. Load all 8 images with the same projection into the Viewer Window.
6. Check the correspondence of orthophotos from the different times. Do this by clicking on the check mark for the top image while zoomed in to alternatively display the bottom image.
7. Create a map of the 2004 orthophotos and a map of the 1999 orthophotos. Be sure to include the elements of a good map (scale bar, north arrow, label, legend, etc.). Place the maps in your G:/share folder.

7. Pick out several (three or four) linear features such as rivers, roads, or railroads and measure their lengths between two interesting points. Indicate the features and their lengths on the overlay.
8. Pick out several features such as lakes, wetlands, or developed areas and measure their areas. Indicate the areas on the overlays.
8. Find an area of at least 1 square mile that has changed significantly between 1999 and 2004, e.g. forest to pasture, forest to suburb, pasture to forest, or some other sequence of changes. Measure the size (length, width, area, etc.) of the changes. You will have to think up a way to measure the differences.
9. Create a proper map illustrating the area that was changed.
10. Finally, write a short report (< 2 pages) about how your reprojected the images (include the parameters), how you measured the differences, and what the results of the measurements were.

Turn in to G:\share\student share\GEO 4120c\yourfolder\ or e-mail to Dr. Binford mbinford@geog.ufl.edu