LoS Tools

QGIS plugin to create and analyse lines-of-sight. Provides also set of supporting tools. The plugin is focused on point to point visibility, in contrast to classic viewshed analysis which is point to area. Besides that it provides tools for identification of horizon points (on LoS), horizon lines (from a set of LoS) and also additional useful tools related to visibility analysis.

Plugin is available here: https://plugins.qgis.org/plugins/los_tools/

Important terms

Line-of-sight (LoS)

Line from a specific observer point (observer) that to or through specific target point. This plugin defines three types of LoS: local, global, without target.

The local and global LoS are used to describe visibility of specific target (i.e. highest point of building, top of the hill etc.) from observing points. Local LoS is strictly between observer and target and does not allow to assess the visibility behind the target. Global LoS is from observer through target but continues behind it as much as underlying elevation raster allows. Global LoS allows assesment of visibility of target with respect to surface located behind it.

LoS without target is a special case of LoS where the target point serves only as descriptor of direction and is not used to evaluate visibility at all. The target points for this can be generated by tools Create points around and Create points in direction. Sets of LoS generated using these tools can be used to identify horizon lines.

Horizons

Generally, horizon is a point on LoS that is visible and is immediately followed by an invisible point. There are several types of horizons mentioned in the plugin's tools. Mainly local and global horizons and horizon lines.

Global horizon is a horizon that is highest on LoS and if the surface raster extent is sufficient it should be the point where LoS meets the sky. LoS have just one global horizon.

Local horizon is any horizon on LoS that is not global. There may be any number of local horizons on LoS. Amogst these local horizons the highest local horizon can be useful for some analyses. The maximal local horizon is the highest horizon that is not global.

DEM use in the plugin

Most tools in the plugin allow you to use more than one DEM raster. The rasters are ordered from smallest cell size to the biggest. The cell size of each raster must be unique to form complete ordering of rasters. Elevation is always obtained from raster with smallest cell size that has valid elevation value (no data is not considered as valid value). This allows utilization of DEM with large cells as addition to DEM with smaller cells. For example DEM with cell size 1 meter can be used within city for areas close to observers and SRTM data (30 meter cell size) in areas around the city, far from the observer.

LoS Sampling strategy

For tools Create Global LoS, Create Local LoS, Create No Target LoS the LoS is determined by sampling distance, which determines how often a point is placed on the LoS, and optionally by maximal LoS length.

# Create No Target LoS V2 parametrizes LoS by so called Sampling distance - distance table. This approach is significantly more flexible, as it allows the sampling distance to change based on distance from the observer. The table is output of tools Calculate Object Distances and Calculate Object Sizes. Each row specifies distance from which specific sampling size should be used. For example row with column values of Distance (meters) 1000 and Size of object (meters) 1.754 specifies that sampling distance of 1.754 meters should be used on LoS when the length exceeds 1000 meters. Maximal length of LoS is given by maximal value of Distance (meters).

There can be two special rows in the table with values of Distance (meters) 0 and -1. Row with value 0 specifies sampling distance for distance smaller than smallest provided distance (default value is 1). Row with value -1 means that there is no maximal LoS length and that the limit is provided by raster data extent.

Citation

The citation for the plugin should be:

Jan Caha (2023). LoS Tools. QGIS Plugin version 1.0. https://jancaha.github.io/qgis_los_tools/

@Manual{,
  title = {LoS Tools}. QGIS Plugin version 1.0,
  author = {Jan Caha},
  year = {2023},
  url = {https://jancaha.github.io/qgis_los_tools/}
}