Terrain mesh introduction

Definition

Mesh is the name given to the computer model of the surface of the earth- The surface over which the user flies when in the simulator, and the surface onto which the users lands (or crashes!)

• Words with similar meaning include: DTM (Digital Terrain Model), DEM (Digital Elevation Model) or 'Elevation model'.

• All scenery objects are placed in relation to the mesh but differ as to whether they are placed with an absolute Z value or simply placed 'on' the terrain. Due to the fact that mesh data can be replaced with alternate data, consideration should be given to whether scenery objects are placed relative or absolute on the terrain and steps taken to avoid objcts floating/sinking above/below the mesh.

• Prior to FSX, mesh was "2.5D" or quasi 3 dimensional (I.e. Latitude X and Longitude Y but with vertical Z values extending upward from an infinite plain at 0). Users couldnt truly fly 'around the globe' but were able to fly continuously across it- The simulator would continue to load data tiles across the world to give the illusion of flying around the earth.

• With the release of FSX, a fully three dimensional mesh was introduced meaning that flights at high altitude would show the curvature of the earth and users could zoom out to see the whole globe modelled. This was termed 'round earth' in the marketing material and introduced some issues with scenery design.

Creation of Mesh

It is not practical for the casual user to create their own mesh- The majority of mesh creation is, in fact, conversion of existing GIS data into a BGL

Conversion of mesh uses the resample tool which converts a number of common DTM formats into a Flightsim BGL

Other Scenery Elements that Effect Mesh

• Flatten- These override the mesh in the area covered by the polygon and replace the height values in order to raise or lower the terrain to a defined, known level such as an airfield.
• Water Polygons- Similar to a Flatten but with a water effect and information about slope added.
• Vector Line data (I.e. Roads, Streams etc) These flatten the terrain underneath the line in a number of ways (defined in Terrain.cfg) including removing slope accross the line (i.e. a road), raising the terrain under the line by a set amount (I.e. Railway) or lowering it by a set amount (I.e. Stream)