Grid Modelling
Whilst it's possible to create a grid model in n4ce, using a reference model to calculate the heights at regular spacing, this is stored in the normal xyz format, namely as an irregular network of points. There is no efficiency in storing points in this way.
The earliest form or DTM was a grid, since it was very efficient in storing and retrieving height data. Gridded height information has always been available through OS Land Profile data and more recently through Lidar. Both use a regular spacing of data in X and Y directions making it possible to store heights in a simple two-dimensional array. Mass data storage is therefore possible and eases access since the data is already indexed. The downside is round off, since data is only collected at these regular spacings.
Both triangular and grid DTMs can be used for volume and section calculations. In the case of gridded volumes, a simple projection applies since break lines and boundary recognition are not possible. It must be remembered that there is another penalty to accept since the originating data will only be measured to a precision of say +/- 10cm.
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The Designer edition has functions for importing and editing Gridded data. Currently there are three distinct gridded formats recognised by n4ce, namely Lidar ASC and Land Form Profile in both DXF and LFP formats.
Having selected the file to import you will be presented with the dialog shown opposite. This identifies the extents of the model and invites you to dissect the model to maximise efficiency.
In some cases, the gridded model may be along strip, for example along a watercourse or the route of a pipeline. This would cause empty cells to appear in the model compromising efficiency. By adjusting the cell size to suit the width of the model, multiple grid models will be formed thus minimising blank cell storage.
The Image button allows you to add an ECW format file to the project, which can be used as a Backcloth and superimposed onto the 3D model, as shown apposite. |
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The model graphics editor for grids in Designer has its own menu bar and tools, shown above, and has been specially created to handle gridded models. The normal 3D model and Dedicated CAD Backcloth icon options are available as is a new Query random point icon.
Long, Cross and Radial sections are available and can be associated with normal triangular DTMs. A feature introduced with sight lines prioritises models using normal triangles in preference to grids in a particular area, if available.
A DTM edit function has been introduced allowing the grid model to be lowered or raised within polygon boundaries. Further, the grid DTM can be overlaid on top of another triangle/grid model and levels re-interpolated. The editor also provides a crop function, allowing you to reduce the size of your model. Finally, an export function allows you to create external files in, Lidar format, that can be loaded into other packages hec-ras and ISIS for water flow model calculations.
In practically all other aspects, the grid model is the same as the triangle model with the exception of simple prismoidal calculations and the omission of groups. Below is an application that provides lines of site using a combination of both Grid and Triangular DTMs.
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