Graduate School of Human and Environmental Studies, Kyoto University

Examples of Geomorphometric Analysis (Geomorphometry) Using mapRaster2

By Noboru Ogata


The following pictures show the results of the processing geomorphological data using computers. Computer models of the landform are called Digital Elevation Models (DEM) or Digital Terrain Models (DTM). Although there are several ways of modeling landform, the commonest way of modeling is regularly sampled elevation on the rectangular grid (Grid DEM). DEMs are used not only for bird’s eye views of landform, but also for quantitative analyses of landform using computers including derivation of geomorphological features such as gradient and aspect angle. The following examples are of 50-meter grid DEM data from the “Sanjô-ga-take” quadrangle in Nara Prefecture supplied by the Geographical Survey Institute of Japan. The study area includes “Sacred Sites and Pilgrimage Routes in the Kii Mountain Range” inscribed on the World Heritage List of UNESCO in July, 2004. Variables are calculated from the DEM data and draped over the surface models in color.


General Landscape

Satellite imagery of LANDSAT TM is draped over the surface model. The imagery was acquired on June 1, 1989. The draped image of true color composite is of band 1, 2 and 3. This picture shows that the study area is mostly covered by forests.

 

Elevation

The color denotes the elevation.

 

Gradient

Gradient angle at each location was calculated from the DEM and draped in color.

Procedures of calculating slope parameters such as gradient and aspect angle are implemented in mapRaster2. Source cocde showing equivalent processing is shown in the link below.

Source code in C++ language ... slope.txt

 

Aspect Angle

Aspect angle of the slope at each location was calculated from the DEM and draped in color. Zero (blue) denotes the north-facing slope and the value increases clockwise up to 360 in degrees.

New! : A new color scale is added so that a circular phenomenon such as the aspect angle can be appropriately depicted. In this color scale, minimum and maximum values are both shown as blue. In this picture, the slope facing north is painted in blue and the slope facing south is painted in yellow (September 23, 2014).

 

Convexity / Concavity

Convexity and concavity (negative convexity) of the terrain are calculated via Laplacian operation. In the study area, where rainfall, surface runoff and streams are dominant factors of terrain formation, index of convexity / concavity shows ridge lines and stream lines. To extract major features of the terrain, the original data are preprocessed by smoothing operation.

Procedure of Laplacian operation is very simple. Please learn the procedure using the following source code in C++ language ... mrproc.txt

 

Plan Curvature

Zevenbergen and Thorne (1987) proposed a procedure to output surface parameters such as slope angle, aspect angle, profile curvature and plan curvature through calculating coefficients of the quadratic surface (polynomial) fitted to the nine neighborhood points. The result of calculating plan curvature is shown on the right. As for the result of aspect angle, same values are output in every 180 degrees...(hey!) Since the grid of the processed DEM is assumed to be square, the DEM is preprocessed through resampling.

Source code in C++ language ... quad.txt

 

Ridge Lines

Ridge lines are shown in yellow. The function of extracting ridge lines from DEMs is not implemented in mapRaster2 yet. The program for this function was coded by students at our laboratory using C++ language, based on the procedure described in the pages 54-55 of Principles of Geographical Information Systems for Land Resources Assessment by P. A. Burrough (1986).

Source code in C++ language ... ridge.txt

 

Stream Lines

In the same way as shown above, stream lines are extracted from the DEM and shown in blue. Because of the nature of the Grid DEM, stream lines in the real world cannot be traced exactly. Therefore extracted stream lines are sometimes disconnected.

Source code in C++ language ... drain.txt

 

Hydrological Modeling

Surface flow from each point on the DEM is simulated, then the flow courses are accumulated to map stream lines. As mentioned above, because of the nature of the Grid DEM, simulated stream lines are sometimes disconnected. More important difference of this model from the reality is that, on the forested surface such as this study area, rainfall mostly infiltrates into the soil and does not run over the surface.

Source code in C++ language ... stream2.txt