Source of the Data
The Terrain Awareness feature uses SRTM (or Shuttle Radar Topography Mission) data that has been specially formatted for use with Tag Pilot.
The SRTM data was collected in 2000. As of late 2015 the highest level of detail (30 meters/pixel) was made publicly available. This is a great example of how NASA's work helps people.
From USGS: "Endeavour orbited Earth 16 times each day during the 11-day mission, completing 176 orbits. SRTM successfully collected radar data over 80% of the Earth's land surface between 60° north and 56° south latitude with data points posted every 1 arc-second (approximately 30 meters)."
We are using the "SRTM 1 Arc-Second Global" variant. This data is available in the USGS EarthExplorer system.
With this new capability comes a few more things to think about when planning a mission. Due to limitations in the aircraft, the DJI SDK, the source data and real world obstacles any terrain aware flight should be carefully reviewed and verified before taking off.
Return To Home - When the RTH button is pressed on the remote or any other event that triggers a Return-to-Home occurs, the aircraft will ascend to the currently set Return-to-Home height if it is higher than its current altitude and proceed in a straight line to the takeoff location. The thing to consider here is that if, for example, you fly up and over a mountain and you are down on the other side back near your currently set return to home height and an RTH event occurs, the aircraft will fly in a straight line back to the takeoff location and directly into the mountain. This shouldn't be done because the flight would violate line of site requirements but it is still possible to send your aircraft there.
To combat this, all terrain aware flights will automatically adjust the Return to Home height to be 60 meters higher than the highest altitude encountered in the current flight path (if possible). Even if the aircraft is right next to the takeoff location when this occurs and is only 60 meters over head, it will ascend to higher than the highest point, come home and descend.
TL;DR - Don't fly up and over stuff because the return to home function wasn't really designed for these kinds of scenarios and there isn't anything Tag Pilot can do about it.
Maximum Flight Height - DJI limits the height an aircraft can fly to 500 meters with respect to the takeoff point.
Source Data Limitations - 30 meter pixels are HUGE. This is the highest resolution elevation data that is available. Look around you and imagine that everything was turned into flat 30 meter squares. There is certainly some data loss.
Date of Collection - This data was collected in the year 2000. If a mine pit was dug before that it may show up. If it was taken after that, it will not.
Unrepresented Obstacles - A utility pole, for example, will not show up. Skyscrapers may show up but the 30 meter granularity will cause issues for the accuracy of the height of the building and the area surrounding it. A vertical cliff face will be located plus or minus 30 meters. These are examples of vertical features that will not be well represented due to the size of the data samples and how it was collected.
Missing Data - 99.9999999% of the supported area is well covered. The data is MOSTLY fine. There are areas where the where the radar was unable to get a reading that have "voids". These empty data locations are in areas where the data has not been fully processed by NASA and are generally in areas of deep ground features or glaciers. In the presence of a "void", Tag Pilot will attempt to use the last known good elevation. This is the safest course of action since these voids usually occur in deep places the aircraft shouldn't be going to anyway. The voids are usually small and Tag Pilot attempts to overestimate the altitude.