Add Elevation Column

[V999TEC]

To see signs of Tropospheric Propagation it is very handy to know the calculated elevation angle of the signal.
It is straightforward to calculate the angle providing the site elevation above sea level is known, the distance of the aircraft and the height of the aircraft.
This fork of the code changes 4 files only.
It needs to persist 1) the site elevation in local storage and 2) a negative angle is stored in local storage, used as a threshold. The latter is used so the user can define the angle compatible with regular "flat" atmospheric conditions.
The current code highlights the aircraft icon by flashing (every other second) when the aircraft goes below the threshold angle set. It actually toggles between its current colour and (typically) red.
Unless the site is at sea level, one would still expect to see some aircraft at negative elevation angles (even when there are no signs of Tropospheric Propagation). Any airfield lower than the receiving site is likely to indicate negative angle as long as it is not too far away. Obviously the curvature of the earth is significant. Having a configuarble threshold is useful.
Each site will be able to determine a default negative angle to represent "flat" propagation conditions.
By default -0.5 is assumed, but each site will differ, especially if there are nearby airfields.
What is fascinating to wtness under "lift" conditions is both aircraft beyond the regular radio horizon (naturally resulting in negative angles) but also aircraft within the normal range but very much lower in height than would normally be observable. The highlighting of aircraft below the configured threshold will give an excelent impression of where ducting is presently occuring.

[eric1tran]

I've seen requests for this. Will take a look at see if this change makes sense

[V999TEC]

I did a POC and left a few details here: https://sites.google.com/view/add-elevation/
Have been running with this code for some time.
There are two basic features 1) the extra filter column 2) the highlighting of "over the horizon" a/c
The latter need not necessarily be implemented and it was a bit hacky to get the icons to flash, but it suited my purpose and was kind of fun.