# Combine properties

Combine properties, categories, or descriptors using expression to calculate new properties and display them in the table view. The descriptor needs to be located at the same level in the Analyse Tree as the properties, categories, or descriptors that are supposed to be calculated.

## Parameters

### Expression

Write the expression for which properties to combine.

Standard arithmetic (`+`

,`-`

,`/`

,`*`

…) and comparison operators (`=`

,`>`

,`<`

…) as well as some mathematical expressions (…) and constant values can be used in expressions.

The spatial parameters that can be used:

`Area`

`Length`

`Width`

`Circumference`

`Regularity`

`Roundness`

`Angle`

`D1`

`D2`

`X`

`Y`

`MaxBorderDistance`

`BoundingBoxArea`

For details on each available property see: Object properties Details

You can also use descriptors such as classification or quantification models. To make sure that no conflict is created always use an alias on the descriptors you want to use.

User-defined categories or properties using “Add Variable Id” can also be used just to write the header of the categories/properties.

Other expressions that can be used are bands or wavelengths to create you can find examples of them in the example section.

### Type

**Object**Applies the expression on objects.

**Pixel**Applies the expression on individual pixels.

### Smooth

Smooth prediction result using median filter kernel

**None**No Smoothing prediction.

**Low**Smoothing using median filter kernel with 5x5 pixel box

**Medium**Smoothing using median filter kernel with 10x10 pixel box

**High**Smoothing using median filter kernel with 15x15 pixel box

## Examples

### Properties

**Expression:**

`(property1 + property2) * property3`

### Spatial descriptor

**Expression:**

`circumference / area`

`(y / length) + (x / width)`

### Band

Indexing with 1 based index in band list. If band name can also be used instead of band number

**Expression:**

`(b10 + 1.5)/ b15`

#### Wavelength

Get band by using wavelength number

**Expression:**

`(w980 + 1.5) / w1024`