![]() Occasionally, contours may cross, appear to intersect, or form an unclosed branching line. A large cell size may result in coarse, blocky contours. The size of the raster cells used affects the appearance of the output contours. Overall contour accuracy depends on how well the data used to create the input raster represents the actual surface. The contour tools produce high-quality contours, representing an exact interpretation of the raster surface. How the 830 contour line is calculated Contour quality The contour line is constructed by joining these intersection points.Įxtrapolation is not used to interpolate the outer corner values, and therefore the contour lines do not extend to the edge of the raster. To create the 830 contour, linear interpolation between the points is used to determine where that value falls along the appropriate vertical, horizontal, and diagonal lines. Once that is done, the values of the existing cell centers and the new intersection locations are used to determine the path of particular contour values. For the group of four cells at the upper left, this value is calculated as follows: (799 + 802 + 825 + 828) / 4 = 813.5, which is then rounded to 814. The first step is to use bilinear interpolation to calculate a central value for each group of four adjacent cells. To understand how contour lines are created, consider the example of a raster of nine cells, with the values at the cell centers as shown below, for which you want to create a contour line for an elevation of 830 meters. The contour attribute table contains an elevation attribute for each contour polyline. They correspond with the areas of higher elevation (in white on the input elevation dataset). The areas where the contours are closer together indicate the steeper locations. The example below shows an input elevation dataset and the output contour dataset. ![]() Contours are also a useful surface representation, because they allow you to simultaneously visualize flat and steep areas (distance between contours) and ridges and valleys (converging and diverging polylines). Why create contours?īy following the polyline of a particular contour, you can identify which locations have the same value. ![]() The contour creation tools, Contour, Contour List and Contour with Barriers, are used to create a polyline feature dataset from an input raster. Where the values rise or fall rapidly, the lines are closer together. Where there is little change in a value, the lines are spaced farther apart. The distribution of the contour lines shows how values change across a surface. Some examples are isobars for pressure, isotherms for temperature, and isohyets for precipitation. Contour lines are often generally referred to as isolines but can also have specific terms depending on what is being measured. The line features connect cells of a constant value in the input.
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