Active remote sensing

As the next step, we should select some data from the Data Sets tab. There are two distinct types of remote sensing based on the type of sensor: active and passive. In active remote sensing, we emit some kind of signal from the instrument and measure its reflectance from the target surface. We make our measurement from the attributes of the reflected signal. Three very typical active remote sensing instruments are radar (radio detection and ranging) using radio waves, lidar (light detection and ranging) using laser, and sonar (sound navigation and ranging) using sound waves. The first dataset we download is SRTM (Shuttle Radar Topographic Mission), which is a DEM (digital elevation model) produced with a radar mounted on a space shuttle. For this, we select the Digital Elevation item and then SRTM. Under the SRTM menu, there are some different datasets from which we need the 1 Arc-Second Global. Finally, we push the Results button, which navigates us to the results of our query. In the results window, there are quite a few options for every item, as shown in the following screenshot:

The first two options (Show Footprint and Show Browse Overlay) are very handy tools to show the selected imagery on the map. The footprint only shows the enveloping rectangle of the data, therefore, it is fast. Additionally, it colors every footprint differently, so we can identify them easily. The overlay tool is handy for getting a glance at the data without downloading it.

Finally, we download the tiles covering our study area. We can download them individually with the item's fifth option called Download Options. This offers some options from which we should select the BIL format as it has the best compression rate, thus, our download will be fast.