Re: [Flightgear-devel] Helipad thresholds

[James T. <ja...@fl...>]

> On 18 Jul 2023, at 22:24, Florent Rougon <f.r...@fr...> wrote:
> 
> Because I spent some time clearing this up due to the somewhat strange
> lat/lon data in LIPX.threshold.xml, here is a comment regarding these
> *.threshold.xml files (example from TerraSync's LFPG, possibly
> outdated):
> 
>    <threshold>
>      <lon>2.61060285</lon>
>      <lat>48.99875734</lat>
>      <rwy>26R</rwy>
>      <hdg-deg>265.41</hdg-deg>
>      <displ-m>577.0</displ-m>
>      <stopw-m>0.0</stopw-m>
>    </threshold>
> 
> AFAICT, and despite the <threshold> element name, the 'lat' and 'lon'
> coords should indicate where the runway _end_ resides (i.e. where you
> _start_ when taking off from 26R... it's called this way in the apt.dat
> v1000 spec) — not the runway threshold. 'displ-m' is the distance from
> that runway end to the corresponding threshold. So, changing displ-m has
> no effect whatsoever on the startup position when using --runway=26R.

That’s because, the displaced threshold is for landing, not taking-off (and hence, starting position)

> 
> One thing I find strange, though, is that I can't use the built-in
> launcher as in the attached screenshot in order to start above the
> runway threshold: this puts me above the runway end. Maybe I use this
> incorrectly?

We could add a startup option which is ‘at the threshold’ but again my understanding, received from other more-aviation-trained people than myself, is that displaced thresholds are about landings; for takeoffs you can always use the full paved area as we describe it. (Maybe there is some concept of a ‘displaced start position’ but we don’t encode it anywhere right now)

Kind regards,
James