The **Dimension** options allow you to add dimensions to selected points. The dimensions that are calculated relate to reference model and when these options are selected, you will be asked to select this model using the normal DTM selection dialog. The dialog items in the **Parameters **group of this dialog will vary depending on the chosen option and will be described below.

These options all follow the current pick mode, so you will be asked to either indicate points, sketch a rectangle or to confirm a list operation.

## Heights

The first two **Dimension** options, **Interpolate** and **Ht Difference**, will interpolate a height from the reference DTM and use this to create a dimension. The name of that dimension is defined by the **Dimension **field in the dialog used to select the DTM. When using a triangular DTM, the **Group Depths **checkbox can be used to adjust the interpolated height. If this is checked and the picked triangle is part of a group, the interpolated height will be reduced by the group depth. No dimension will be assigned if a triangle cannot be picked from the reference DTM. Note that if a point is coincident with a point in the reference model and the triangles using that point have different groups assigned, the first triangle that is encountered will be used.

The **Interpolate** option allows you to store the interpolated height as a dimension. The **Ht Difference **option will store the difference in height between the indicated point and the interpolated value. Therefore, for this option to work, you will only be allowed to choose points that have a valid height.

## Gradients and Directions

The **Dimension Gradient & Direction **option allows you to calculate these values from the reference DTM. You cannot choose the dimension names and the gradient will always have a name of *G= *whilst the direction dimension will be the same as that used for the angle dimension for text and symbols, normally *A=*. The value of the gradient is expressed as a percentage and the direction is always in decimal degrees. The direction that is calculated is the polar angle where the positive x-axis is zero and anti-clockwise is positive. Note that if a point lines of the common edge of two DTM triangles, the first triangle that is encountered will be used.

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