- 1 From Topographic Maps to Digital Elevation Models Which Way Does the Water Flow A topographic map shows the relief features or surface configuration of an area. A hill is represented by lines of equal elevation above mean sea level. Contours never cross. Elevation values are printed in several places along these lines.
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From Topographic Maps to Digital Elevation Models Which Way Does the Water Flow A topographic map shows the relief features or surface configuration of an area. A hill is represented by lines of equal elevation above mean sea level. Contours never cross. Elevation values are printed in several places along these lines.
Riek, Dave, News Director has reference to this Academic Journal, PHwiki organized this Journal From Topographic Maps to Digital Elevation Models Daniel Sheehan DUE Office of Educational Innovation & Technology Anne Graham MIT Libraries Which Way Does the Water Flow A topographic map shows the relief features or surface configuration of an area.
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A hill is represented by lines of equal elevation above mean sea level. Contours never cross. Elevation values are printed in several places along these lines. Contours that are very close together represent steep slopes.
Widely spaced contours or an absence of contours means that the ground slope is relatively level. 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. Maps in mountainous areas may have contour intervals of 100 feet or more.
A city can be overlain on a topographic map. A bench mark is a surveyed elevation point. Contour lines point up stream.
United States Geological Survey Topographic Map Symbols Explained http://erg.usgs.gov/isb/pubs/booklets/symbols/ Digital Elevation Models Using elevation data in raster as long as mat in a GIS
What is a Digital Elevation Model (DEM) Digital representation of topography Cell based with a single elevation representing the entire area of the cell Basic storage of data DEM as matrix of elevations with a uni as long as m cell size Adding geography to data Xmin, Ymin XY are in projected units Xmax, Ymax Cell index number x cell size defines position relative to Xmin, Ymin in addition to Xmax, Ymax in addition to infers An exact location
Uses of DEMs Determine characteristics of terrain Slope, aspect Watersheds drainage networks, stream channels Scale in DEMs Scale determines resolution (cell size) Depends on source data Resolution determines use of DEM in addition to what spatial features are visible Estimating slopes in a DEM Slopes are calculated locally using a neighborhood function, based on a moving 33 window Distances are different in horizontal in addition to vertical directions vs diagonal Only steepest slopes are used cell size
Slopes (elevations) (difference/distance) Flow Direction Useful as long as finding drainage networks in addition to 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 Flow direction in a DEM Flow directions as long as individual cells
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 Watersheds Contour lines (brown) Drainage (blue) Watershed boundary (red) Once done manually
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 Flow accumulation in a DEM Flow accumulation as long as individual cells Flow direction in a DEM Flow directions as long as individual cells
Flow accumulation as drainage network Drainage network as defined by cells above threshold value as long as region. Things to do be as long as e the exercise: In Windows, create a new folder under your username on the F:Usertemp folder. In Arcgis Desktop, click on tools then Extension. Check the box as long as Spatial Analyst in addition to close the window. Again click on tools in addition to then Customize. Again, check the box as long as the Spatial Analyst toolbox in addition to close the window.
Riek, Dave News Director
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