[pdal] Syntax for implementing new filters.hag relative to TIN

[Andrew Bell]

On Fri, Nov 8, 2019 at 11:18 AM Andrew Cunliffe <andrewmcunliffe at gmail.com>
wrote:

> Can anyone help with the syntax to calculate HAG (height above ground)
> relative to an interpolated surface (following James McClain’s
> contribution, #2520 <https://github.com/PDAL/PDAL/pull/2520>)?
>
>
> I’ve tried:
>
> {
>
>     "type": "filters.hag",
>
>     "delaunay": True
>
> },
>
>
>
> But the output HAG is clearly still being computed relative to the nearest
> neighbour, rather than an interpolated surface as desired.  I’ve tried *"delaunay_fan":
> True*, and also including *“count”: 4* arguments, and have been looking
> through GitHub (which led me to “delauney” rather "delaunay_fan") but still
> haven’t succeeded.
>

The option is "delaunay", as you've specified, though the constant 'true'
must be specified in lowercase to be valid JSON.

> What am I missing?
>
Looking at the code, it appears that you also need to set "count" to some
reasonable value in order to get this interpolation method to work.  The
code builds a delaunay triangulation of <count> neighbors and then
interpolates attempts to use that triangulation to determine a Z value for
the point in question.  I'd have to look through the math more closely to
give you better information.  It appears that things haven't been
documented.  I'll take a stab at that before long.

-- 
Andrew Bell
andrew.bell.ia at gmail.com
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