From Topographic Maps to Digital Elevation Models

1. From Topographic Maps to Digital Elevation Models Daniel Sheehan Office of Educational Technology & Innovation (dsheehan@mit.edu) Anne Graham MIT Libraries (grahama@mit.edu)

2. Which Way Does the Water Flow? All the maps I�m showing you are scanned images of paper maps like these. This is a quick look at these maps for those of you who haven�t seen them before. Daniel will then talk to you about digital elevation models and let you play with them.All the maps I�m showing you are scanned images of paper maps like these. This is a quick look at these maps for those of you who haven�t seen them before. Daniel will then talk to you about digital elevation models and let you play with them.

3. A topographic map shows the relief features or surface configuration of an area. It is taking the 3-D world we live on and illustrating it in two dimensions�.a map. Note the many symbols and wording.It is taking the 3-D world we live on and illustrating it in two dimensions�.a map. Note the many symbols and wording.

4. Round Hill looks like a series of consecutive circles. The other hills� contour lines don�t cross even though they aren�t round. Round Hill looks like a series of consecutive circles. The other hills� contour lines don�t cross even though they aren�t round.

5. Elevation values are printed in several places along these lines.

6. Contours that are very close together represent steep slopes.

7. Widely spaced contours or an absence of contours means that the ground slope is relatively level.

8. The elevation difference between adjacent contour lines, called the contour interval, is selected to best show the general shape of the terrain. A map of a relatively flat area may have a contour interval of 10 feet or less.

9. Maps in mountainous areas may have contour intervals of 100 feet or more.

11. A bench mark is a surveyed elevation point.

12. Contour lines point up stream. The water is flowing from the left side of the map to the right, toward the river.The water is flowing from the left side of the map to the right, toward the river.

13. United States Geological Survey Topographic Map Symbols Explained http://erg.usgs.gov/isb/pubs/booklets/symbols/

14. Which Way Does the Water Flow? The contour line crossing the Westfield River at the top of the map is pointing upstream. Therefore, the water is flowing from the top of the map to the bottom of the map in this case.The contour line crossing the Westfield River at the top of the map is pointing upstream. Therefore, the water is flowing from the top of the map to the bottom of the map in this case.

16. Digital Elevation Models

17. What is a Digital Elevation Model (DEM)? Digital representation of topography Cell based with a single elevation representing the entire area of the cell

18. Basic storage of data

19. Adding geography to data

20. Uses of DEMs Determine characteristics of terrain Slope, aspect Watersheds drainage networks, stream channels

21. Scale in DEMs Scale determines resolution (cell size) Depends on source data Resolution determines use of DEM and what spatial features are visible

22. Estimating slopes in a DEM Slopes are calculated locally using a neighborhood function, based on a moving 3*3 window Distances are different in horizontal and vertical directions vs diagonal Only steepest slopes are used

23. Slopes

24. Flow Direction Useful for finding drainage networks and drainage divides Direction is determined by the elevation of surrounding cells Water can flow only into one cell Water is assumed to flow into one other cell, unless there is a sink GIS model assumes no sinks

25. Flow direction in a DEM

27. Finding watersheds � Begin at a source cell of a flow direction database, derived from a DEM (not from the DEM itself Find all cells that flow into the source cell Find all cells that flow into those cells. Repeat � All of these cells comprises the watershed The resulting watershed is generalized, based on the cell size of the DEM

28. Watersheds �

29. Flow accumulation The number of cells, or area, which contribute to runoff of a given cell The accumulation function determines the area of a watershed that contributes runoff to any given cell

30. Flow accumulation in a DEM

31. Flow accumulation as drainage network