When photographing in urban environments, for example New York city, it's often the case that buildings or other man-made structures can obscure your view of the sun.
TPE for iOS includes a number of tools to help you establish the position of the sun or moon relative to a building of known height.
However, the simplest tool is to understand the length of a shadow cast by a building. The first step is to go to the Shadows and Elevation page and enter the height of the building of interest (you can usually easily find the heights of prominent buildings via a web search, e.g. "Empire State Building height"):
You should place the primary pin (red) on the location of the building and select the required date and time. Then, when you change the time of day by panning the altitude chart beneath the map, TPE shows the shadow length cast by the building:
Another way of interpreting this is to say that, if you were to stand at the end of the shadow line at this time, then the sun would appear directly at the tip of the Empire State Building. TPE shows "lunar shadows" too, but of course there are not visible during daylight, and might only be marginally detectable during anything other than full moon at night. However, it's a useful tool to know where to stand to position the moon atop a building in your shot.
NOTE: one important caveat regarding shadow lengths - they are calculated assuming flat ground. If you're standing on top of a hill looking down at a building, then all bets are off - the shadow will fall on the side of the hill and would be significantly shorter than indicated on the map.
You can also use the geodetics tools in TPE to see when the sun or moon is obstructed by a building. To try this option, first enable geodetics (tutorial here), place the grey pin at your shooting location and then swap the pin positions to place the red pin at your shooting location and the grey pin on the Empire State Building.
When you press the 'swap pins' button, the building height is associated with the grey pin (as the Empire State Building didn't actually move when you moved the red pin...). The geodetics chart shows the sun and moon altitude (in degrees above the horizon) relative to the angle of view to the top of the building:
You can see from the screenshot above that both sun and moon are obstructed at this time of day. (You'll need to confirm the alignment visually using the map - in this example, the moon is lower in the sky than top of the Empire State, but it lies slightly to the north of the building, as you can see from the blue line direction on the map.)
You can double tap the chart to check the elevation mode instead, but remember that the building height indicator extends much farther than visible scale of the chart in this instance:
The dotted lines indicate that both sun and moon lie below the height of the building top.
Optionally, you can apply a manual correction or offset to the reported elevation data used by the app. This may be desirable in areas where the accuracy of the elevation data is poor.
Elevation data is obtained from web services, principally Google Elevation and GeoNames SRTM3. While usually accurate, you may find the data is less than perfect in certain areas. (One example recently found is Golden Cap in Dorset, England, where the reported max elevation is some 100 feet lower than the true elevation.)
For shots requiring critical alignments, always double check the reported elevation data against a known reliable source, e.g. USGS topographic maps in the USA, or Ordnance Survey maps in the UK, and apply corrections using the elevation offset fields.
Any elevation offsets you enter are applied as a weighted average along the path from primary to secondary pin when showing the elevation or altitude profile.