Grid Data - Output (XYZC Data)
The Output page in the Grid Data dialog includes output options for the grid file, including geometry, data limits and transforms, and generating a report.
Specify grid file output options in the Output page
Output Grid Geometry
The Output Grid Geometry section defines the grid limits and grid density.
Copy Geometry
The Copy geometry from option copies the grid geometry from an existing map layer or grid file. This option is useful if you have many grids that you'd like to all use the same geometry.
To copy the geometry from an existing layer, select the layer in the Copy geometry from list. To copy the geometry from a grid file, click Browse and select the file in the Open Grid dialog. Select <None> to return the Output Grid Geometry options to their default values and to manually edit the grid geometry.
Minimum and Maximum X, Y, and Z Coordinate (Grid Limits)
Grid limits are the minimum and maximum X, Y, and Z coordinates for the 3Dgrid. Surfer computes the minimum and maximum X, Y, and Z values from the XYZC data file. These values are used as the default minimum and maximum coordinates for the grid.
Grid limits define the X, Y, and Z extents of the output grid. The extents of the grid define the extents of contour maps, color relief maps, shaded relief maps, vector maps, 3D wireframes, and 3D surfaces created from grid files. When creating a grid file, you can set the grid limits to the X, Y, and Z extents you want to use for your map. Once a grid file is created, you cannot produce a grid-based map larger than the extent of the grid file. If you find you need larger grid limits, you must regrid the data. You can, however, read in a subset of the grid file to produce a map smaller than the extent of the grid file.
When either the X, Y, or Z value is in a date/time format, the date/time values are converted and stored in the grid as numbers.
Spacing and # of Nodes (Grid Density)
Grid density is a measure of the number of grid nodes in the grid. The direction (X Direction, Y Direction, or (Z Direction) that covers the greater extent (the greater number of data units) is assigned 100 grid nodes by default. The number of grid nodes in the other directions are computed so that the grid nodes Spacing in the three directions are as close to one another as possible.
By defining the grid limits and the number of rows and columns, the Spacing values are automatically determined as the distance in data units between adjacent rows and adjacent columns.
Note on High Density Grid Files
Higher grid densities (smaller Spacing and a larger # of Nodes) increase the smoothness in grid-based maps. However, an increase in the number of grid nodes proportionally increases the gridding time, drawing time, and the grid file size. You can have up to 2,147,483,647 rows and columns in a grid file. It is likely your computer will run out of memory before reaching the maximum grid size. The primary use for the large grid size maximum is to allow grids with extreme aspect ratios to be created.
More information on grid file density can be found here.
Grid C Limits
In some cases, the gridding interpolation and extrapolation can result in undesired values, for example negative numbers in cases where negative values are physically impossible. The Grid C Limits options clamp the grid output to specific minimum and maximum values.
The Grid C Limits are applied after the interpolation operation. After the grid interpolation is performed, Surfer locates any grid values less than the Minimum and replaces them with the Data min or Custom value. Surfer locates any grid values greater than the Maximum and replaces them with the Data max or Custom value.
To clamp the output to a specific minimum value, click the current selection next to Minimum, and select None, Data min, or Custom from the list. If Data min is selected, the data minimum will be displayed in the field to the right of the Minimum list. Select Custom and type a value in the input box to use a user-defined Minimum.
To clamp the output to a specific maximum value, click the current selection next to Maximum, and select None, Data max, or Custom from the list. If Data max is selected, the data maximum will be displayed in the field to the right of the Maximum list. Select Custom and type a value in the input box to use a user-defined Maximum.
Assign NoData
2D Convex Hull
Select 2D convex hull from the Assign NoData outside of list to assign the NoData value to the grid nodes outside the 2D convex hull of the data. The 2D convex hull of a data set is the smallest convex polygon containing all the data. The 2D convex hull can be thought of as a rubber band that encompasses all data points. The rubber band only touches the outside points. Areas inside the 2D convex hull without data are still gridded. See the Grid Data topic for an example of assigning NoData outside the 2D convex hull of the data.
The convex hull is a 2D polygon. When calculating the 2D convex hull of data for an XYZC data set, only the XY coordinates are considered
If 2D convex hull is selected from the Assign NoData outside of list, the Grid Data dialog displays the Inflate 2D convex hull by box. Enter a value to expand or contract the 2D convex hull. When set to zero, the boundary connects the outside data points exactly. When set to a positive value, the area assigned the NoData value is moved outside the 2D convex hull boundary by the number of map units specified. When set to a negative value, the area assigned the NoData value is moved inside the 2D convex hull boundary by the number of map units specified. Values are in horizontal (X) map units. If the value is set to a large positive value, the grid values may extend all the way to the minimum and maximum X and Y limits of the grid, essentially overriding the Assign NoData outside 2D convex hull of data option. If the value is set to a large negative value, the entire grid may be assigned the NoData value, resulting in no grid file being created.
Alpha Shape
Select Alpha shape from the Assign NoData outside of list to assign the NoData value to the grid nodes outside the alpha shape of the data. Select Alpha shape instead of 2D convex hull to have a tighter boundary, especially if a boundary could form concave areas. If Alpha shape is selected from the Assign NoData outside of list, the Grid Data dialog displays the Alpha value box. Enter a value to define the radius of the circles that are created by the edges of the triangulation of the points. Larger alpha values create a more convex boundary and smaller alpha numbers create a tighter, concave boundary. If too low or too high of an alpha value was entered, a message will appear stating that the grid could not be assigned NoData because of the alpha value. For more information on alpha shapes or to determine an appropriate alpha value, see the Alpha Shape help topic.
C Transform
The C Transform option changes how the C values are gridded. Available options are Linear; Log, save as log; and Log, save as linear. To change the C Transform option, click on the existing option and select the desired option from the list.
Linear uses the C values in the worksheet for gridding. No transformation is applied to the C values. The Linear method is a good option for data that gradually increases over space. This is the default C Transform.
Both Log options use take the log (base 10) of the C values before gridding. The log (base 10) of the C value is then used for gridding. The Log, save as log takes the log (base 10) of the C values and uses the log value for gridding. The grid is then saved with the log (base 10) values. The Log, save as linear takes the log (base 10) of the C values and uses the log value for gridding. The grid is then converted back to the linear C values by taking the antilog of the gridded results. When Log, save as log or Log, save as linear is selected, at least three data points must be positive C values. Negative values are ignored for gridding. Both Log methods are good options when the data changes very quickly over a small area or when very high and very low values occur very closely to each other. This can be common with concentration values in ground water or geochemical data.
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The images above display the difference in gridding the posted data with linear (top right) and log (bottom contours). The log contours on bottom show the difference in C values between the grid when Log, save as log (bottom left) and Log, save as linear (bottom right) are selected. |
NoData 2D Polygon Boundary
Specify the region or regions to be assigned the NoData value and whether or not to use only selected objects in the NoData 2D Polygon Boundary section. Select either a map layer or vector file in the NoData 2D Polygon Boundary section:
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Click the current selection and select a base layer from the list. Only base layers that contain at least one eligible polygon or polyline will be included in the list. The base layer must use the same source coordinate system as the grid.
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Click Browse to load a vector file with the Open dialog. The file must use the same coordinate system as the grid.
The number of polygons and vertices is displayed below the NoData 2D polygon boundary once a file or map layer has been selected. If the boundaries have blanking flags or BLN_Flag attributes, the total number of inside and outside flags is displayed.
Note that all boundaries are treated as 2D polygon boundaries when assigning NoData meaning that only the XY coordinates are used when determining which grid nodes will be assigned the NoData value. For 3D grids, this means that the same XY boundary will be used to assign NoData at all Z levels.
Polyline Boundaries
2D and 3D polylines can be used for NoData polygon boundaries. The polylines in the base layer or vector file will be treated as polygons and 3D polylines treated as 3D polygons while assigning NoData values. The Assign NoData command is not recommended with open polylines as unexpected results may occur. Before clicking Grids | Edit | Assign NoData, consider converting the object to another type better suited to the operation you wish to perform with one of the Change To commands, and edit features with the Reshape command.
If the layer you wish to use contains both polygons and polylines, but you only wish to use some or all of the polygons, select the objects you wish to use before clicking Grids | Edit | Assign NoData and select the Selected objects only option. If the file you wish to use contains both polylines and polygons, first load the file as a base layer, and then use the Assign NoData command with the Selected objects only option.
Inside, Outside, or Mixed
Select Inside to assign the NoData value to the region inside the NoData polygon/3D polygon boundary or boundaries. Select Outside to assign the NoData value to the region outside the NoData polygon/3D polygon boundary or boundaries. Select Mixed to use the blanking flag or BLN_Flag attribute values from the file or layer. The Mixed option is only available when the layer or file contains both blanking flags or BLN_Flag attributes: assign NoData inside (1) and assign NoData outside (0). If all blanking flags or BLN_Flag attributes are the same, the NoData Inside or NoData Outside option is selected automatically, and the Mixed option is not available.
Selected Objects Only
Select the Selected objects only option to use only the selected objects in the base layer to assign NoData values to the grid. When the Selected objects only box is checked, the Loaded polygonsandvertices values are updated. Select a base layer in the NoData 2D Polygon Boundary field to use the Selected objects only option. The Selected objects only option is not available when the NoData 2D Polygon Boundary is a vector file. The polygon or polygons must be selected before clicking the Grids | Edit | Assign NoData command.
Supported Vector Formats
The following file formats can be used to assign NoData values to a grid based on a polygon boundary:
Output Grid
Choose a path and file name for the grid file in the Output Grid group. You can type a path and file name, or click the 
button to browse to a new path and enter a file name in the Save Grid As dialog.
Grid Report
Check the box next to the Grid Report option to create a gridding report that includes all the gridding parameters used to generate a grid. This report also includes statistics about the grid. You can also access the grid statistics by creating a grid information report. Create a grid information report in the Grid Editor by clicking the Grid Editor | Options | Grid Info command or by clicking the Grids | Info | Grid Info command from any document window.
Add New Map or Layer
Check the Add grid as layer to check box to automatically add the created grid to a new or existing map. Select [New Map] in the Add grid as layer to field to create a new map. Click the current selection and select an existing map to add a new layer to the map. Select the layer type by clicking the current selection in the New layer type field and selecting the desired layer type from the list.
Save Settings
Click Save Settings to save the current Grid Data dialog settings from all dialog pages (Select Data, Options, and Output) to a Grid Data Settings file. Grid Data Settings files can be loaded on the Select Data page when creating grids.
What's Next?
Click Finish to create the grid file, or click Back to return to previous dialog pages.