GPS for Dummies.pdf - Engineering Surveyor

Chapter 14: From the Air with USAPhotoMaps231If you plan to use USAPhotoMaps to build an extensive aerial photo collection,a high-speed Internet connection will make your life much easier. Eachindividual aerial photo or topographic map image in the displayed mosaic isroughly 50–200K in size. (The actual size depends on how much unique terrainis shown in the image.) At 2 meters-per-pixel resolution, it takes 70 imagesto fill the screen of a 1024 x 768 monitor; that’s anywhere from 7–17MB of datato download.Getting the Most from Aerial PhotosAfter you have an aerial photo displayed, navigate inside the image. (Readthe preceding section for how to download and display an aerial photo.)USAPhotoMaps has a number of different commands to view and manipulatethe photo.The datum used for all TerraServer-USA maps is NAD 83. This is nearly identicalto the WGS 84 datum used as a default by GPS receivers. If you plan onusing your GPS receiver with USAPhotoMaps, be sure that the datum settingsmatch. (Read more about datums in Chapter 2.)The first thing you’ll notice is that when you move the mouse cursor aroundthe screen, map coordinates appear in the window title bar. This feature allowsyou to move the cursor over a feature that’s shown on the aerial photo andget its precise coordinates.You can change how you want the coordinates displayed from the View menu.Choose the Lat/Lon item and then specify whether you want the coordinatesshown in decimal degrees, degrees and decimal minutes, degrees/minutes/seconds, or UTM formats.Press the X key to display a dialog box with the coordinates that the cursor iscurrently over. You can copy the coordinates from this dialog box and pastethem into another program.Switching between aerial photosand topographic mapsAfter you know where you are in a photo (see the preceding section), lookat the commands that you can use to move around. As I mention earlier,TerraServer-USA provides both aerial photos and USGS topographic maps,