Point Clouds and Polygons

Productive Meshes from GIS for 2D Upstream Landuse Modelling

A computational mesh which describes critical terrain features efficiently is an essential ingredient of a numerical simulation to map and diffuse peaks of high magnitude events.

The method below is a development of the point cloud structure adopted to describe the Upper Severn Reach from Munlyn to Buttington. It is published here as an ‘open’ iterative step towards procedure which

for any UK catchment upstream of communities at risk
describes effectively terrain & amenity infrastructure to engineer exceedance flows
supports riparian frictional vegetation (eg lateral and longitudinal porous ‘speed-bumps’) grown in software,
resolves numbers productively (budget ‘TIN‘= 10-15 points/hectare average)
handles model boundaries efficiently (ie simple equations/time-sets for main & tributary discharges),
calibrates to real/surveyed events,
supports online flythrough publication in real-time (for good communication to stakeholders)

For a basic 100-1000 hectare GIS, by points (for River 2D finite element engine) :-

Load and crop the national Z3 (Q100) floodplain perimeter; save as area;
Digitize centre line (Cline); buffer (for bank lines) to main channel width; extend by 100m at inflow, outflow and main tributaries
Create lateral and longitudinal attenuation features (Lats & Longs) from bank-lines (BlineL, BlineR) to Z3 perimeter; save as Lo & La
Buffer combined Lo & La (LL) in proportion to main channel and floodplain (eg LL8 and LL32)
Split LL8 and LL32 (LL8L, LL8R, LL32L, LL32R) and resample to points @ 32m
Resample Cline @ 32m
Buffer Z3 by 32m (z332)
Combine Z332 and LL32 and convert to points @ 64m (= model boundary)
Raise flow ridges (LL ‘speed-bumps’) and lower Cline to bathymetry
Buffer Z332LL32 by 32m for Extents and resample @ 64m (inflow, outflow and tributaries by boundary equation between Extent and Boundary)
Append polygon to ‘extents’; cut vee notch ‘canyon’ between inflow and outflow.
Superimpose terrain point cloud over ‘vee-notch-canyon’ (ie retain simple productive inflow, tributary and outflow boundary conditions)
This should contain and describe the terrain features to budget, but within reach of real-time resolution at 2GHz