What is it?
The term "Geodetics" means data derived from the field of Geodesy, "the scientific discipline that deals with the measurement and representation of the Earth (or any planet), including its gravitational field, in a three-dimensional time-varying space".
In TPE, it's not actually as complicated as that makes it sound.
Why do we need it?
Primarily to determine sight lines between camera and subject.
For the purposes of photo planning, there are some situations where we can't get the accuracy we need by being "flat earthers": we need to take the curvature of the earth into account. For example, a tall mountain is made apparently smaller with distance (simple trigonometry gives us the results we need here), but the curvature of the earth makes it even lower on the horizon than a "flat earth" calculation would give us. Spherical trigonometry can take that into account.
However, the Earth is not a sphere - it's an ellipsoid. To do the job properly, we need to use mathematics derived from geodesy. (Thank you, Thaddeus Vincenty.)
How do I enable it?
Geodetics is enabled by pressing the enable geodetics button on the map, as shown below:
When enabled, the geodetics tool provides key information for the path between red and grey map pins. You can use this information to help plan your shot (more details below).
How do I exit Geodetics?
Tap the 'back' button as shown:
What does it tell me?
When enabled, the distance, bearing, difference in elevation and altitude angle are shown, as measured from primary (red) to secondary (grey) pin.
In addition, the elevation or altitude profile between the two map pins is displayed. This helps to visualize the terrain and possible line of sight between camera and subject. Once the elevation profile data is received, the altitude display is changed to show the maximum altitude (in degrees) along the path.
The chart shows the following information:
- The primary (red) pin corresponds to the left of the chart
- The secondary (grey) pin corresponds to the right of the chart
- Terrain that is visible from the primary pin position is indicated by a solid line
- Terrain that is obscured as viewed from the primary pin position is indicated by a dashed line
- The sun or moon are shown on the chart if they lie within 45° of the bearing from primary to secondary pins. The line are shown with transparency if the azimuth is greater than 2.5° difference. If the sun or moon would be obscured, the line is shown as dashed
The chart supports the following gestures:
- Tap once to toggle scale-to-fit on off.
- A Double Tap to switch between elevation and altitude profile on iPad, or to cycle between elevation, altitude and sun/moon altitude charts on iPhone
The altitude chart displays the altitude angle along the line from primary to secondary pin. In this mode, the sun/moon altitude lines appear horizontal. The altitude chart can provide a better representation of the relative size of objects as viewed from the primary pin. For example, mountains that are taller, but farther away, appear in the correct relative size on the altitude chart, but always appear taller on the elevation chart.
One easy way to think of the altitude chart representation is this: imagine the y-axis at the left of the chart is the plane of your camera's sensor or film. Looking out along the x-axis is a cross section of what your camera will see from the red pin position.
If the sun or moon are obstructed by the terrain, you'll see a dashed line as shown:
TPE uses elevation data from both Google Elevation Service and the Shuttle Radar Topography Mission (SRTM30). Data points are typically spaced between 30m and 90m (295ft) apart. If the distance between the map pins is less than the data spacing, a warning icon is displayed indicating that there may not be sufficiently granular data available to calculate accurate results.