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.
數字高程模型（DEM）在生物物理環境的空間分析中有着廣泛的應用。爲了繪製實驗室活動的農業生態區地圖，我們將使用DEM計算坡度（山坡的陡度）和坡向（山坡面對的方向）以及其他地形衍生物
我們提供給您的DEM（tcdem_utm83）面積比我們湯普金斯縣的研究區域大。因此，我們可以使用一個覆蓋湯普金斯縣本身範圍的多邊形（LULC_2012_utm83）作爲“cookie cutter”來刪除縣外我們不需要的數據。如果我們較大的（餅乾麪糰）數據集是矢量格式的，這個過程將被稱爲“剪切”。使用較大的光柵數據集，這個過程被稱爲“掩蔽”
從Tompkins_County地理數據庫中，將大型DEM（“tcdem_utm83”）添加到Lab4.mxd中。如果需要，請重新排列TOC中的圖層，以便縣邊界顯示在DEM的頂部。（另外，請確保在Lab4.mxd ArcMap文檔（上面的步驟1.6）中激活了空間分析員擴展）。
要使DEM的大小和範圍與湯普金斯縣完全相同，請使用ArcToolbox>空間分析工具>提取>按掩碼提取。輸入光柵爲“tcdem_utm83”，輸入光柵或特徵遮罩數據爲“LULC_2012_utm83_e2”。將輸出光柵設置爲“TC_elev_utm83”，並確保它將進入您的湯普金斯縣地理數據庫。
