Digital elevation models (DEMs) are used extensively in spatial analyses of the bio-physical environment. For mapping our agro-ecological zones for our lab activity, we will use a DEM to compute slope gradient (steepness of a hillslope) and slope aspect (direction a hillslope is facing), among other terrain derivatives
The DEM that we have provided you (tcdem_utm83) covers an area larger than our Tompkins County study area. So we can use a polygon that covers the extent of Tompkins County itself (the LULC_2012_utm83) as a “cookie cutter” to remove the data outside of the county that we do not need. If our larger (cookie dough) data set were in a vector format, this process would be called “clipping.” Working with a raster data set as the larger one, this process is called “masking.”
From your Tompkins_County geodatabase, add the large DEM (“tcdem_utm83”) to your Lab4.mxd. If needed, re-arrange the layers in your TOC so that the county boundary is displayed on top of the DEM. (Also, make sure the Spatial Analyst extension is activated in your Lab4.mxd ArcMap document, which was Step 1.6, above).
To make a DEM exactly the size and extent of Tompkins County, use ArcToolbox > Spatial Analyst Tools > Extraction > Extract by Mask. The Input raster is “tcdem_utm83” and Input raster or feature mask data is “LULC_2012_utm83_e2”. Make the Output raster “TC_elev_utm83” and ensure it is going to your Tompkins_County geodatabase.
数字高程模型（DEM）在生物物理环境的空间分析中有着广泛的应用。为了绘制实验室活动的农业生态区地图，我们将使用DEM计算坡度（山坡的陡度）和坡向（山坡面对的方向）以及其他地形衍生物
我们提供给您的DEM（tcdem_utm83）面积比我们汤普金斯县的研究区域大。因此，我们可以使用一个覆盖汤普金斯县本身范围的多边形（LULC_2012_utm83）作为“cookie cutter”来删除县外我们不需要的数据。如果我们较大的（饼干面团）数据集是矢量格式的，这个过程将被称为“剪切”。使用较大的光栅数据集，这个过程被称为“掩蔽”
从Tompkins_County地理数据库中，将大型DEM（“tcdem_utm83”）添加到Lab4.mxd中。如果需要，请重新排列TOC中的图层，以便县边界显示在DEM的顶部。（另外，请确保在Lab4.mxd ArcMap文档（上面的步骤1.6）中激活了空间分析员扩展）。
要使DEM的大小和范围与汤普金斯县完全相同，请使用ArcToolbox>空间分析工具>提取>按掩码提取。输入光栅为“tcdem_utm83”，输入光栅或特征遮罩数据为“LULC_2012_utm83_e2”。将输出光栅设置为“TC_elev_utm83”，并确保它将进入您的汤普金斯县地理数据库。
