Filter Expressions

A number of commands use a filter expression to control the data included in the output of a process. Some commands provide controls for building an expression directly in their dialog box, while others provide a field to launch the Expression Builder dialog box, which contains the expression building controls.

Regardless of the tool used, filter expressions can be built using the operator buttons in the dialog box and values entered through the keyboard. The operators supported in the command are listed below.

Operator	Type	Function	Example
( )	Grouping	bracketed operation	(Depth+1)
+	Mathematical	addition	Depth + 4
*	Mathematical	multiplication	Depth * 4
–	Mathematical	subtraction	Depth - 4
/	Mathematical	division	Depth / 4
^	Mathematical	exponentiation (Depthx)	Depth ^ 4
<	Mathematical	less than	Depth < 4
>	Mathematical	greater than	Depth > 4
=	Mathematical	equal to	Depth = 4
!=	Mathematical	not equal to	Depth != 4
AND	Logical	conditional relation	Depth > 4 AND Uncertainty < 0.5
OR	Logical	conditional relation	Depth > 4 OR Density > 5

Multiple expressions can be combined to build conditional expressions in the form below, where each part (shown in bold type below) can be a full expression in itself:

Conditional expressions provide the added ability to have no‑data values in the results. When no‑data values are assigned to nodes in the output, those nodes will not be displayed and cannot be selected or exported.

Conditional expressions also allow mathematical expressions in the results. This allows the attribute values to be altered differently for each result or additional expressions to be run based on the results of the first expression. Examples of both are shown below:

If (Intensity> -46 AND Intensity< -25.4) Then 1 Else If (Intensity> -24.399 AND Intensity< -19.2) Then 2 Else If (Intensity> -19.199 AND Intensity< -14.5) Then 3 Else If (Intensity> -14.5 AND Slope > 8.51) Then 4 Else If (Intensity> -14.5 AND Slope < 8.51) Then 5 Else NO_DATA

If using the AND operator, both relations must be true for the complex expression to be true. If using the OR operator, only one of the relations must be true for the expression to be true.

• When defining conditions, if, then and else must always be entered in the order that they are specified here.

• When a part of the expression (Expression, ResultA, ResultB) contains multiple parts, each part of the expression must be surrounded by brackets.

• The correct case must be used when specifying names of 'Available attributes', for example Deep and Density.

• When filtering a coverage based on a contributor band, and the contributor band values include file paths, the expression must be entered as the full URI.

Scenario	Expression	Result
Checking the depth and uncertainty values of all data on a surface.	If (Depth > 10 AND Uncertainty < 0.2) Then Depth Else NO_DATA	Each node that is deeper than 10 and has uncertainty less than 0.2 is added to the new layer using a value from the input Depth layer. All nodes that do not meet the criteria will be added to the new layer with the NO_DATA value.
Testing a variable resolution surface against IHO Order 1a.	If (Density >= 5 AND ((Depth<=40 AND Resolution<=1) OR (Depth>=40 AND Resolution <= (0.1 * Depth)))) Then NO_DATA Else 1	Each node that meets the criteria is added to the new layer with the NO_DATA value. All nodes that do not meet the criteria will be added to the new layer with a value of 1.
Identifying all data in a coverage that originated from a specific contributor dataset.	Contributor='file:///C:/Surfaces/CUBE_05m_Final.csar'	All data that originated from the CUBE_05m_Final.csar coverage is saved to a new coverage.
Creating a filtered point cloud using LAS classification data.	Classification = 'Ground' OR Classification = 'Low Vegetation' OR Classification = 'Medium Vegetation' OR Classification = 'High Vegetation'	All data classified as ground or vegetation is included in the new point cloud.