Discover can produce a gridded surface from any mapped data, which has a numeric column representing a z-value or grid channel. Examples of this type of data include topographic spot heights, soil geochemical sample results or
geophysical data such as ground magnetic, or gravity data. Discover can also grid line and polygon data providing there is an associated numeric z-value. Therefore it is possible to create a grid directly from data such as contour lines without having to pre-process the data first and convert the line information into points.
Exercise 8.1 Create an elevation grid from spot height data over the Darlot area
1. Open the DARLOT_SPOT table from the Encom
Training\Darlot\Topography folder and using the Information Tool, query
a few of the spot heights to see the associated data. We will create a grid using the AHD_RL field. The elevation values range from a low of 429.31 to a high of 528.6m. You can check this by selecting Query > Calculate
Statistics and selecting the DARLOT_SPOT table and the AHD_RL field.
2. Select Discover>Surfaces menu. A new Surfaces menu should appear on
the MapInfo Menu Bar. Select Surfaces>Create Grid and select the DARLOT_SPOT layer or right-mouse click on DARLOT_SPOT layer in the
ELC and choose Create Grid.
3. The elevation data is loaded into the Discover Gridding Tool. This utility is
controlled via a series of tab pages to the left of the preview window. On loading the data Discover has automatically computed a set of gridding parameters, however we can modify these according to the data to be gridded.
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4. Select AHD_RL from the list of numeric fields available to grid.
Input Tab
The Input tab summarises the input data and provides a selectable list of all numeric data columns that are available for gridding in the source Tab file or query. Basic summary information is provided for the selected data including the number of points, minimum and maximum field values and minimum and maximum easting (X)/northing (Y) values for the minimum bounding rectangle that fully encloses the input data.
A Breakline table can be selected in order to force gridded data to conform to certain slope requirements in critical areas, eg drainage traces, topographic ridge lines or roadways.
The Coincident points pull-down list contains a number of options to use in the event that there are two or more data points at the same location.
The Data Conditioning option enables you to condition or annotate the data used to grid. The input grid data can be clipped to specified extents, null values can be excluded from the gridding process or converted to background values and minimum and maximum capping values can also be set.
Statistics Explorer
The Statistics Explorer is a utility for examining and understanding the spatial and statistical aspects of a dataset. The Statistics Explorer will be examined in more detail in the next exercise.
Preview Window
View input points in the preview window. Click button to remove input points from view.
View search ellipse in the preview window. Only available if Inverse Distance Gridding method is selected.
Apply a histogram equalisation stretch to the image in the preview window. This button can be used to distribute colours more evenly across the image and is particularly useful for data with poor dynamic range.
42 Encom Discover 3D Workshop Manual
Display images in the preview window as either coloured or monochrome (black and white). This button toggles the image between monochrome and colour. Apply a sun illumination to the image in the preview window. This can be used to enhance detail within a gridded image. The sun angle is fixed from the north-east direction.
Interrogate the quality of the interpolated surface using the Statistics Explorer. Select a look-up table to colour the grid. The chosen colour table will be applied to the gridded image when loaded into Discover.
A pop-up menu can also be accessed by right-clicking with the mouse in the preview window:
5. Click on the Method tab and select Triangulation as the method.
The Triangulation method produces a regular gridded surface through a set of data points by using an optimised Delaunay triangulation algorithm. The triangular mesh is created by drawing lines between adjacent input data points and forming an irregular network such that no triangle edges are intersected by other triangles. A regular grid is then computed from the triangular irregular network and grid cell values computed using a natural neighbour interpolation process. As the original data are used to define the triangles, this method is very useful for situations where the data must be honoured very closely (eg elevation data in a digital terrain model).
The triangulation method is best applied to data that is evenly distributed over the gridded area. If there are large areas of sparse or missing data distinct triangular facets may appear in the output grid. As triangulation uses all the input data to construct the triangular mesh, the only parameter that needs to be adjusted is the grid cell size. Grid cell size can be adjusted to an appropriate value for the dataset.
6. Click on the Geometry tab and leave the default Cell size at 150m.
7. Choose the Output tab and select ERMapper as the output grid format.
Save the grid to the same folder as the input data points and click Save to create the grid and open it into a new map window. Turn off the visibility for the DARLOT_SPOT layer in the map window.
To report grid cell values directly to the screen, choose the Grid Info button from the Surfaces button bar.
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8. To view information about the grid select the Surfaces>Grid Information
menu option.