GPS for Dummies.pdf - Engineering Surveyor

270Part III: Digital Mapping on Your ComputerFigure 16-6:Topographicmap combinedwithDEM datashowingfeatures andelevation.You can change the size of the map by choosing Resize Overhead View fromthe Operation menu. This brings up the Map Scale dialog box, where you canuse the slider bar to change the image size. Entering a 1 for the Map Scalevalue displays the map at a 1:1 ratio. You can move around the map withscroll bars if it’s too big to fit in the window.So what’s with the numbers in the DRG filename?Why couldn’t the USGS just have named it afterthe name of the topographic map (Mount St.Helens) to make things easier? There’s actuallysome logic behind the file-naming convention ofDRG map files (called the o-code, short for Ohiocode).Carve the o46122b2 filename to seewhat it means: 46: 46 degrees of latitude. 122: 122 degrees of longitude.DRG filenames b: A row within that latitude and longitudesquare. (Rows run south to north and arelabeled a–h.) 2: A column within the latitude and longitudesquare. (Columns run east to west and arenumbered 1–8.)That makes a little more sense. All USGS DRGsuse this standard, which makes it easy for programsto figure out which data files should beused for displaying adjacent maps.