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ForestERA Data Layer Details - Slope / Aspect (in degrees)

Note to data users: Please carefully review the metadata provided with each layer. We request that users consult with the ForestERA project in advance of using these data in publications and/or presentations to ensure that the strengths and limitations of the data are considered.

Description

These are 90m resolution derivatives from a digital elevation model (DEM) of the western Mogollon Plateau in Arizona. The 7.5-minute digital elevation model data are digital representations of cartographic information in a raster form. The DEMs consist of an array of elevations for ground positions at regularly spaced intervals. The data are produced in 7.5- by 7.5-minute blocks and stored as profiles with a 10 or 30-meter square grid spacing along and between each profile. The DEM data for 7.5-minute units correspond to the USGS 7.5-minute topographic quadrangle map series for all of the United States and its territories except Alaska. The slope and aspect data layers are derived directly from the DEM and have the same resolution and spatial extent. Both layers are provided here in units of degrees, although slope may also be calculated in percent. For certain types of modeling (e.g., wildlife habitat) aspect should be converted into two separate grids by converting aspect from degrees to radians (degrees / ?) and taking the sine and cosine of the resultant grid (Beers et al. 1966). This procedure converts the aspect layer from a circular variable to two layers that have continuous values between 1 and -1. The sine function divides aspect into a north-south component, while the cosine function divides aspect into an east-west component.

Purpose

This data layer was obtained by the ForestERA project to support landscape-scale forest restoration planning efforts by a broad group of stakeholders including federal and state agencies, academic institutions, and non-governmental entities. These data are intended for regional analyses over spatial extents on the order of tens to hundreds of thousands of acres, and were not developed for use at finer spatial scales, although they may be useful for some applications at finer scales.

Development

Digital Elevation Models are created by the United States Geological Survey (USGS) using interpolation techniques and one of two elevation data sources; either digitized cartographic map contour overlays or scanned National Aerial Photography Program (NAPP) photographs and specific ground locations where elevation has been precisely measured. See the USGS web site describing Digital Elevation Models (http://edc.usgs.gov/guides/dem.html) for more specific information on the methodologies used in their creation. We obtained 30m resolution DEM mosaics for several regions from the Southwest ReGAP project (see regional DEM layers at http://www.gis.usu.edu/docs/projects/swgap/data/archive/grid.html). We combined these mosaics together using standard methodologies to obtain a continuous grid surface across the study area and resampled the original 30m resolution grids to 90m to match the resolution of our vegetation data.
Methods for deriving slope, aspect, and other terrain attributes are standard in ArcView, ArcGIS, and many other GIS platforms and can be determined by Methods for deriving additional terrain attributes have also been developed and published (e.g., Jones et al. 2000).

Accuracy Assessment

The Digital Elevation Model and all of its derivatives meet USGS standards for accuracy. The USGS assesses the accuracy of DEM data by comparing linear interpolation elevations in the DEM with corresponding map location elevations and computing the statistical standard deviation or root-mean-square error (RMSE) for all differences between the two. See the USGS web site describing Digital Elevation Models (http://rockyweb.cr.usgs.gov/nmpstds/demstds.html) for more specific information on accuracy standards and the methodology for accuracy assessment. For the DEMs used in this project, 90 percent of the predicted elevation values are expected to lie within 23 feet (7m) of the actual value and the remaining 10 percent are expected to lie within 50 feet (15m) of the actual value.

Sources of errors

The elevation value for any pixel in a DEM is the predicted average value over the entire pixel. Larger pixel sizes lead to more generalization in the elevation values in addition to any errors that may be present. Digital Elevation Models should not be used to determine exact elevation at scales less than the size of the pixel.

Recommendations

We recommend that this layer be used at a minimum resolution of 90m (approximately 0.8 ha or 2 acres) for purposes of analysis and display. In addition, the user should review USGS guidelines at the web pages mentioned above for additional information on the use of digital elevation models and their derivatives.

References

Jones, K. B., D. T. Heggem, T. G. Wade, A. C. Neale, D. W. Ebert, M. S. Nash, M. H. MeHaffey, K. A. Hermann, A. R, Selle, S. Augustine, I. A. Goodman, J. Pederson, D. Bolgrien, J. M. Viger, D. Chiang, C. J. Lin, Y. Zhong, J. Baker, and R. D. Van Remortel. 2000. Assessing landscape condition relative to water resources in the western United States: a strategic approach. Environmental Monitoring and Assessment 64: 227-245.

Beers, T. W., P. E. Dress, and L. C. Wensel. 1966. Aspect transformation in site productivity research. American Scientist 54: 691-692.

Page last updated February 23, 2005