Allow Negative Target Height in Viewshed Analysis (viewshed.py)

[trygvels]

Description:

This pull request introduces support for negative target heights in the viewshed function. The modification ensures that target points can have a decreased z-value, which is particularly useful for cases where visibility analysis needs to account for building facades rather than rooftops.

Reason for Change:

When using the library for viewshed analysis from an observer in a tall building, the function primarily returns visible points on rooftops rather than building facades. Allowing negative target heights makes it possible to adjust the z-value of the target point while keeping all other points treated normally, improving accuracy in urban visibility studies.

Fixes #

Proposed Changes

Modified the conditional check in _viewshed_cpu from: if target_elev > 0: to if abs(target_elev) > 0

• This change ensures that the viewshed function correctly considers negative target heights while maintaining the existing behavior for positive values.