Lea 92 used aspect associated with each pixel to determine the flow direction. “An aspect driven kinematic routing algorithm’”.
Previous studies have recoded aspect into 8/16 different directions so an error of +- 44, 22 degrees is acceptable. (Coops 2000) Visual interpretation of histograms was used to find the difference: X axis we have angles (difference) and on Y axis no of pixels having that difference. Skidmore ‘89 Spearman’s rank correlation coefficient is used to find the correlation between the two aspect maps. Skidmore ’89 Wind direction modeling, habitat suitability, site selection, hydrologic modeling Coarsening the resolution of the elevation grid results is more errors from the truth values of slope and aspect. (Chang – Tsai 1991) The resolution of the data is dependent upon purpose of the analysis
Queens case: The central pixel gets the direction of the steepest slope of eight neighboring pixels, Tarbonton 97 Rooks case: The central pixel gets the direction of the steepest slope of four neighboring pixels
The big slump in the aspect values between 30 to 60 m res is due to the fact that the values are off by just the neighboring pixels. When the pixel is coarsened the aspect value are more conformal.
Comparing directional maps
Rahul Rakshit PhD Candidate Clark University [email_address]
<ul><li>For more information, please contact at : [email_address] </li></ul><ul><li>To tackle other types of variables, read the following from www.clarku.edu/~rpontius: </li></ul><ul><ul><li>Pontius, Thontteh and Chen. 2006. Components of information for multiple resolution comparison between maps that share a real variable. Environmental and Ecological Statistics. </li></ul></ul><ul><ul><li>Pontius and Cheuk. 2006. A generalized cross-tabulation matrix to compare soft-classified maps at multiple resolutions. International Journal of Geographical Information Science. </li></ul></ul><ul><li>Rahul extends special thanks to: </li></ul><ul><li>Prof. Robert Gilmore Pontius Jr. for the concept and advice in every step of </li></ul><ul><li>this research. </li></ul>Acknowledgements and Plugs
Aspect ArcGIS (Radians) Aspect Idrisi (Radians) Cos (Aspect) Sin (Aspect) Cos (aspect) Sin (Aspect) Slope (ArcGIS) Slope * Sin Slope * Cos Aggregate to coarser resolution Vector Addition (ArcGIS) Slope (Idrisi) Slope * Sin Slope * Cos Aggregate to coarser resolution Vector Addition (Idrisi) Scatter Plots at Coarser Resolutions Study Area Mask Mean Absolute Error (MAE) Elevation Coarse Angle(ArcGIS) Coarse Angle(Idrisi) Coarse Angle Difference