ProRaster Help – Contour Levels

On this page, you will find general help for the ProRaster product family including links to documentation, instructional videos, and training videos.
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A contour line is a vector polyline that traces a constant value in a continuously varying raster. A Contour layer can be placed anywhere in a rendering algorithm but will usually be at (or near) the top of the layer stack so that the contour lines render over the top of other raster imagery. Contours are usually associated with terrain (shown below) and bathymetry data, but you can generate contours for any raster data.

ProRaster generates contour lines on-the-fly in a map as you zoom and pan. It uses your computers GPU to render contour lines, so you may realise higher performance if you have an external graphics card. If so, make sure that Windows is making use of the external GPU by checking the System>Display>Graphics settings. 

To get started quickly and easily, drop down the Open algorithm menu button and select “New contour algorithm >” and choose the raster you want to display by any of the available mechanisms. An algorithm is created that displays the raster using either a LUT Color, RGB Color, or Image layer and with a Contour layer on top. Alternatively, add a Contour layer to any algorithm or to an empty algorithm, and then specify the contour properties manually. 

Contours can be rendered using dropout and labels (as shown below). Dropout is used to prevent rendering of minor contour lines in areas where the slope is high, and the contour line density exceeds a threshold. Major contour lines are still rendered and never drop out. Labels are used to annotate the major contour lines. By default, the label will simply be the contour level, but you can use an external file to specify labels manually. Where a line is labelled, the line is not drawn to allow the label to be read. 

Like any layer, you need to specify a raster source for every component of the layer that you enable. By default, only the Contour component is enabled. You must specify a raster source for the Contour component in the normal way, including the field and band and time information. The Contour component is also used to color the contours, if you choose to use color modulation. Just like a LUT Color layer, you need to define a data – color transform and select a color table. The color for each contour level is determined by applying this data – color mapping. 

The opacity component can be used to define the opacity for each contour line, based on the contour level. The contour level is used in the data – opacity transform to acquire an opacity level. Unlike other kinds of layers, you do not need to set the Blending rule in the Algorithm to enable opacity modulation for contours. 

Use the Mask component to prevent rendering of contours where the mask raster that you specify is invalid or zero.

Select the Contour layer in the algorithm tree control and you will see two new property pages – the “Levels” and the “Labels” property pages. On these property pages you specify the contour levels and control the appearance of the contours.

The contour spacing and anchor value are used to compute the contour levels. A contour line will be generated at intervals of the contour spacing. These will be offset from the anchor value. Contour lines will be produced for levels that are an integer multiple of the spacing about the anchor value. For example, if the anchor value is 0 and the spacing is 10, then the contour levels might be …,-30,-20,0,10,20,30,… onwards. If the anchor value is 50 and the spacing is 100 then the contour levels might be …-250,-150,-50,50,150,250,… onwards. 

Contour lines are designated “minor” or “major” where every “Nth” contour will be declared a major contour. Major contour lines will be generated at major step integer multiples of the spacing from the anchor. For example, if the anchor is 0 and the spacing is 10 then major lines will be produced at …,-200,100,0,100,200,… onwards. It is important to note that only minor contour lines are subject to dropout and only major contour lines are labelled. If you set a major contour step of zero, no contour lines will be designated major. 

Check “Automatic Contour Spacing” to let the system assign a contour spacing based on the raster statistics. It will always set the Anchor to zero. The contour spacing will be reflected in the dialog and you can still specify the major contour step. Uncheck this option to define the spacing, anchor, and major step manually. In either case you can also define the first and/or last contour level if you wish the contours to cover some specific range. 

The minor and major contour line widths can be specified in pixels. You can use fractional widths (like 0.5, 1.5 etc). The line width will be multiplied by the “Pixel Scaling Factor” defined in the ProRaster Options dialog. You can also specify the color of the minor and major contour lines as well as the opacity. Note that the opacity is applied regardless of whether the Blending Rule is enabled in the algorithm. 

You can enable minor contour line dropout and set a dropout threshold. When enabled, the system will compute the slope of the raster and in areas of high slope will prevent drawing of minor contour lines. Major contour lines will continue to be rendered. The dropout threshold defaults to 1 and sensible values will range between 0.5 (less dropout) to 5 (more dropout).