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Correcting GIS-based slope aspect calculations for the Polar Regions
| Content Provider | Semantic Scholar |
|---|---|
| Author | Moret, Geoff J. M. Huerta, A. David Velayos |
| Copyright Year | 2007 |
| Abstract | Intr oductionSlope aspect is an important geomorphic parameter . Theaspect of a slope controls its solar irradiation, thus affectinga wide diversity of processes (for a partial review see Mooreet al. 1991). Slope aspect can be calculated from a digitalelevation model using one of several GIS-based algorithms(e.g. Moore et al. 1991, Burrough & McDonnell 1998).These algorithms are designed for mid-latitude regions,calculating slope aspect as an angle relative to grid north. Inpolar regions, however , this approach suf fers from twoproblems:1. In the commonly-used polar stereographic projections,grid north is parallel to either the prime meridian (forthe south pole) or the meridian at 180i (for the northpole) while geographic north varies with longitude.2. In the polar regions, the direction of geographic northcan vary significantly over a relatively small area.As a result, GIS-based slope aspect calculations do not givecorrect values when using polar projections.The method we present here provides a simple techniquefor correcting GIS-calculated slope aspect to determine thetrue, geographic slope aspect. This technique is critical forwork using slope aspect in the polar regions.Aspect corr ectionT o obtain a corrected slope aspect map, we must subtracteach grid cellOs longitude from the GIS-calculated aspect(Fig. 1). In the South Pole S tereographic Projection (seeSnyder 1987, p. 154) the longitude, !, of a data point in thisprojection is independent of the ellipsoid used and is givenby where the four -quadrant arctangent is used. In this equationx and y are the coordinates of the data point after theremoval of false easting and false northing (i.e., when thecoordinates of the pole are 0,0). In the North PoleS tereographic Projection, the longitude is given by ESRI states that incorrect slope aspects in the polar regionsrepresent Oa known limit for our softwareO (personalcommunications, ESRI support, February 2006). Thus, wehave written a Matlab function (Appendix A) to convert theESRI-determined slope aspect into slope aspects that arereferenced to geographic north. This function uses the GIS-calculated aspect data, the grid spacing, and the (x,y)coordinates of the lower left-hand corner of the grid asinputs. The data can be transferred between ArcGIS andMatlab in the form of ASCII files. The Matlab function (Appendix A) allows the user tochoose output ranges of 0i to 360i or -180i to 180i. Thecode also addresses the two special cases of 1) grid cells thatdo not have an elevation in the DEM (ONoDataO cells inArcGIS), and 2) cells that are flat, and thus have no aspect.By convention, the ONoDataO cells are set to a value of -9999 and the flat cells are set to a value of -1. If the userchooses an output range of -180i to 180i the flat cells areset to a value of 9999.AcknowledgementsThis research was partially funded by NSF grant OPP-0534036. W e thank the referee for their helpful comments. |
| Starting Page | 129 |
| Ending Page | 130 |
| Page Count | 2 |
| File Format | PDF HTM / HTML |
| DOI | 10.1017/S0954102007000181 |
| Volume Number | 19 |
| Alternate Webpage(s) | http://www.geology.cwu.edu/facstaff/huerta/HuertaWeb/Site/Publications_files/MoretHuertaAntScience07.pdf |
| Alternate Webpage(s) | https://doi.org/10.1017/S0954102007000181 |
| Language | English |
| Access Restriction | Open |
| Content Type | Text |
| Resource Type | Article |
