The Distance method produces a grid which records the distance from the input data features to each grid node in the output raster. A search distance envelope is specified which restricts (clips) the output grid cell values so they only extend to the limit of the search envelope from the nearest input feature. Any grid node that is outside the search envelope will be assigned a null value in the output raster.
This gridding method will accept point, line, polyline and region vector objects as input. If the input vector data consists of lines, polylines or region objects these will first be converted to point objects by extracting all the vertices from the input objects into a collection of points. These points will be attributed with the attributes from the source object. If you would like to create a distance grid (or buffer grid) around a set of linear objects you may wish to insert some extra vertices into the objects so the point separation is approximately ½ the search envelope distance for best results.
Distance Method Options
- Parameter Unit
Specifies whether spatial parameters are defined in cell units or distance units. The units should be supported by MapInfo Pro, for example, the options for Distance unit include: US Survey feet, yards, rods, chains, miles, nautical miles, millimeters, centimeters, meters, inches, links and kilometers.
Distance Unit - Select a distance unit from the Distance Unit drop-down list.
Click More Options to open the following advanced options.
- Smoothing Method
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You can apply smoothing on the gridded data to produce smoother surface. Smoothing is used to enhance the sharpness of an image or improve the appearance of the edges. Select a suitable Smoothing method and level for your data.
Smoothing Level - Move the smoothing slider to set the smoothing level for the output raster. You can set a value between 0 to 6. A value of zero applies no smoothing. A value of 6 applies maximum smoothing.
- Clipping
- The Clipping control provides options to limit the extents of the interpolated raster, so it more closely approximates the distribution of the input data. Enabling this option can improve the appearance of the output raster when interpolating irregularly spaced input data, where the interpolation methods (such as Triangulation, Minimum Curvature or Natural Neighbor) has interpolated across large gaps in the input data.
- None - No clipping is applied to the raster cells.
- Near Only - The Near value represents the maximum distance from a source input data point for which an interpolated raster cell will be created. Cells in the raster which lie at a distance greater than the Near distance will be assigned a null value. This method has the same effect as applying a distance buffer to the source data points equal to the near distance.
- Near/Far - Interpolated cells in the output raster will be clipped to the near distance if no other data point is found within the Far distance that meet the angular search constraints. Applying both Near and Far clipping can be useful to constrain the interpolated raster to a required distance from the source points. It also allows the larger gaps to be interpolated across in irregularly spaced data.
- Polygon - You can provide a TAB file of polygon(s) to clip the output raster to the polygon boundaries. You can specify whether to clip a region outside or inside the raster bounds. However, it does not support polygons with holes.
- Coincident Points Method
The Coincident Points drop-down list controls the handling of multiple data points at the same location. For more information see, Coincident Point Methods.
- Search Modes
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The search mode defines how the interpolation algorithm searches for input data points within the search neighborhood. The shape of the search neighborhood will be dictated by the type and distribution of the input data. The following search modes are available:
- Spherical - Performs an equidistant radial search within the search neighborhood. The search distance is defined by the Major Axis Search parameter, which is defined in Parameter Units. For optimum performance, it is advisable to keep the search distance small. A value less than or equal to 5x of the output cell size is usually sufficient. Spherical search is best applied when the variation in the spatial distribution of the input data is considered isotropic.
- Elliptical - Performs an elliptical search within the specified search neighborhood. The shape of the ellipse is defined by the Major Axis Search and Minor Axis Search parameters.
A default search radius will be calculated for you. If you wish to enter a specific distance then you can do so by typing the required values into the Major Axis Search box. To apply anisotropic search, choose elliptical search mode and enter a value into the Minor Axis Search and Major Axis Orientation box. The size of the Major axis search must be larger than the minor search axis.
- Major Axis Search
Major Axis Search - The Major Axis Search defines the maximum width (x axis) of the search ellipse and the Minor Axis Search defines the minimum width (y axis). Major Axis Orientation defines the rotation angle of the ellipses Major search axis, measured clockwise from North (0 degrees). An elliptical search is best applied when some directional anisotropy is known to exist in the spatial distribution of the input data.
A default search radius will be calculated for you. If you wish to enter a specific distance then you can do so by typing the required values into the Major Axis Search box. To apply anisotropic search, choose elliptical search mode and enter a value into the Minor Axis Search and Major Axis Orientation box. The size of the Major axis search must be larger than the minor search axis.
Raster Geometry
In the Raster Geometry section, specify the cell size and raster bounds for the output raster. For more information, see Defining Raster Geometry
If required, click More Options to specify category and sub-category of the output projection. If the input file is MapInfo .TAB, projection values are read from the input file, which you can override here.
- Category - The Category drop-down list consists of all projection systems supported by MapInfo Pro. For example, Longitude/Latitude, Universal Transverse Mercator (ED 50), Universal Transverse Mercator (NAD 27 for Canada), etc.
- Sub Category - The Sub-Category drop-down list consists of the type of projection based on the selected projection system.