New Variable Resolution Surface

Source data can often have widely varying point densities over a given area due to depth changes, survey coverage, etc. When using fixed-resolution grids, it can be difficult to choose a single sampling resolution for the resulting raster model that maintains continuity over sparse areas, while preserving details where there is more data.

Variable resolution surfaces are surfaces in which the resolution can vary in different regions, while maintaining continuity across the entire surface. The desired resolution in discrete areas is determined using selected algorithms, instead of by setting a fixed resolution based on subjective choice.

The first step to create a variable resolution surface is to determine how to subdivide the region, and what the resolution of each sub-region (or tile) should be. One option is to select a parameter to define the required resolution in each tile. An alternative is to analyze the source data on several criteria to determine the optimum resolution for each.

The process starts by dividing data into regular tiles, in this case using a quad-tree-structure, recursively dividing binary space partitions until each tile contains the appropriate number of data samples.

Once the data area is subdivided, a resolution for each tile must be determined using a resolution estimation method. The available methods are:

• CARIS Density: which estimates the resolution based on source point density.

• Calder-Rice Density: which estimates the resolution based on point density over an area

• Ranges: which assigns a resolution based on a tile statistic (such as min., max., mean) and a look-up table of resolutions and values.

The resulting quad-tree and computed resolution for each region are stored as the Resolution Map for the dataset. From this point, each tile is populated from the source data using standard gridding methods, such as: inverse distance weighted (IDW) mean, uncertainty-based mean, simple mean or selecting the minimum or maximum value.

Variable resolution surfaces can be created from track lines, raster surfaces, point clouds or TINs. Parameters for creating the surface are set in the New Variable Resolution Surface dialog box.

Each section of the dialog box can be collapsed or expanded using the blue arrow buttons at the right of the fields. By default all sections are collapsed when the dialog box is launched, showing only the fields for each section that require user interaction, all other fields contain default values.

If a setting is changed from the default, the status indicator box to the right of the option field changes to gray. If a mandatory option is not complete, the status indicator box changes to red. Defaults can be restored by clicking on the indicator box and selecting “Reset”.

Option	Description
Input	Set the input data for the new surface.
Source	The input data for the process. If a coverage was selected before launching the dialog box, that coverage is select by default. This can be changed to any raster surface or point cloud currently open in the application.
Options	Select estimation and population methods for creating the surface and set related options.
Estimation Method	Select a method and define the necessary options. Different options are available based on the selected estimation method. The estimation methods available are: • Density (Calder-Rice) to estimate resolution based on point density over coverage • Density (CARIS) to estimate resolution based on source point density • Ranges to assign a resolution based on a tile statistic
Population method	Select a gridding method with which to populate the new surface and define the necessary options. Different options are available based on the selected population method. The population methods available are: • Inverse distance weighting: depth is given by the mean of all samples in the specified neighbourhood, weighted by a function of the inverse of the euclidean distance from the sample to the node. • CUBE: several hypotheses will be calculated based on depth and uncertainty, and the strongest hypothesis returned • Uncertainty: depth is given by the mean of all samples in the resolution bin, weighted by a function of distance and sample uncertainty. • Mean: Use a depth range containing the average value of all points within the tile. • Swath Angle: value set using beam angle and footprint radius that defines the maximum area to which a points will be applied • Selected Value - a single value for the node is used to populate each cell based on the selection criteria set in the options
Output	Define the size of and projection for the surface, and location of the output file.
Extent	By default the extent of the surface will be set to the current zoomed view of the data. (“From data” setting.) Alternatively, select Custom to activate the coordinates fields below. 1. Click the Pick from screen to define a section of the displayed data. This changes the shape of the cursor. 2. Use the cursor to drag a box around an area. The Upper and Lower X and Y coordinates for the area will be updated. To create a surface from the full extent of the data: 3. Click the Use screen extents button. The displayed coordinates will be updated.
Output coordinate system	Click Browse to open the Select Coordinate Reference System dialog box and set the output system. See Change Coordinate Reference System
Output File	Set the name and location of the output surface file.

Menu	Tools > Surfaces > New > Variable Resolution
Tool