It is often useful to be able to show the value and direction of gradients on a model. The Gradient option from the DTM Tools menu allows you to create symbols showing such gradients. When selected, the pop-up menu below will be displayed and there are four options for creation.
Indicate - This option will ask you to indicate any two points within the DTM triangulation. Height values will then be interpolated so that a gradient can be calculated.
Two Points - This option will display the gradient between any two points in the model. You will be asked to indicate two points. Note that points that do not have a valid height will be ignored.
Line Segment - This option will display the gradient of a line feature segment. You will be asked to indicate a feature segment and a gradient will only be displayed if the points at either end of the segment have a valid height.
DTM Triangle - This option will ask you to indicate any point within the DTM triangulation. The gradient and direction will be calculated from the indicated triangle.
Whichever of the four options is selected, the dialog box below will be displayed allowing you to specify the format of the gradient symbol.
The symbol will comprise a line with an arrow head and a piece of text showing the gradient in the direction of the line.
The Direction group allows you to define whether a positive gradient is downhill or uphill. An arrowhead will be placed on the appropriate end of the line. The arrowhead is defined by the Arrow group. The arrow head type can be either closed, an isosceles triangle, or open, a chevron, and is defined in the Type field. The length of the arrow is defined in the Size field with the width at the base of the arrow being assumed to be 0.4 times the length. Note that the length of the arrow should be defined in millimetres.
The Attributes group contains the usual Layer and Pen fields. It also contains the Line Length field for the length of the line and the Line Offset field for the offset of the line. The latter field is only enabled if you are creating a gradient along a line feature segment or between indicated points.
The Text group defines the format of the text showing the gradient. The Style field defines the text style that is to be used and the Decimals field defines the number of decimal places in the gradient value. The Format group allows you to specify whether the text should appear as a percentage or in the 1 in x format.
The positioning of the gradient symbol will depend upon the method of calculating the gradient. If the gradient is from a DTM triangle, the centre of the line showing the gradient direction will be at the centroid of the indicated triangle. If the gradient is calculated from either a feature line segment or two points, you will be asked which side of the two points the gradient symbol is to be positioned. The arrow line will then be offset from the line joining those two points using the Line Offset field. The text will be placed on the opposite side of the arrow line to the indicated points.
The Edit Delete option from the Gradients pop-up menu allows you to delete gradient symbols from the model. You will be asked to indicate the section lines to delete. You should ensure that you indicate an element within the hachuring. The Edit Move option allows you to move gradient symbols. You will be asked to indicate the symbol to move and then give it a new location.