TPE automatically checks the sightlines for rise and set events, such as sunset or moonrise, an indicates if the local topography provides a clear view to the horizon or not.
Note: please read Sunrise (etc.) is incorrect to make sure you're familiar with exactly how rise and set times are defined. The key point to remember is that sunrise is measured relative to the unobstructed horizon.
Note: TPE cannot automatically account for obstructions caused by man-made structures or vegetation (e.g. tall trees). You can manually enter object heights in the shadows screen to check for these types of potential obstructions.
The easiest way to understand the sightline check is with a couple of examples.
Standing on the east shore of Utah Lake looking towards sunset in October, there's a clear view - the land is flat beyond the lake to the west for a sufficient distance that the sun can be seen (weather permitting) as it descends below the horizon:
However, standing in the same place and watching for sunrise, the situation is rather different:
TPE checks the sightline and indicates an obstruction at 8.5km. Looking at the topographic map in the screenshot above, it's clear that the foothills of the Uinta Mountains obstruct the horizon.
If you tap a second time on the sunrise panel on the timeline, TPE offers three options to view the sightline in the direction of sunrise:
On choosing the medium elevation profile, geodetics mode is enabled and the secondary grey pin is dropped at a point along the sightline:
The altitude profile shows clearly the obstruction caused by the foothills which blocks the view of sunrise. The orange sunrise line is shown as dotted, indicating that the view of the sun is blocked.