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Pitfalls in Computing Contour AreasTranslation and free translation from ALS The biggest problem with digitizing contours, even when using a mechanical planimeter, is properly tracing a contour so the contour area is correctly defined. Frequently with maps that have many faults and sections, it becomes difficult to determine where a contour is defined. In the following example (Figure 1) a section has four contours (0’, 10’, 20’, 30’) with a low spot in the lower left corner and a high in the upper right corner.
On first glance, the 10' contour could be incorrectly digitized going around the left corner defining a very small area. The correct way to digitize the 10' contour is to digitize along the contour to the boundary, along the boundary around the upper right corner staying along the map boundary and back down to the starting point. The next figure (Figure 2) shows the area and volume results for the correctly digitized map and the following figure (Figure 3) with the incorrect results. The correct 10’ contour area is 780.2 acres with a resulting Trapezoid volume of 18,095 ac-ft. The incorrect 10’ contoured area was 27.4 acres with a resulting Trapezoid volume of 10,567 ac-ft, a 37.4% underestimate of the actual volume. Figure 2 - Correctly Digitized Map
Figure 3 - Incorrectly Digitized Map (10' Contour reversed)
There are two simple checks to determine if the contours have been property digitized. The first check concerns structure physics. For isopach maps, higher contours must have smaller areas than lower contours. This rule even applies for complicated maps with many highs and lows when contour of equal thickness are added together. In the above incorrect example, the 20' contour has a larger area than the 10’ contour suggesting something was incorrectly digitized. The areas of the contours should always by checked that the areas decrease as the thicknesses increase. Volume calculations of the Modified Trapezoid Rule and the Vertical Slice Method should give you exactly the same results. If the volume results disagree, the user should check that their map has been correctly digitized. Another pitfall in computing contour areas is correctly determining map scales. Maps are frequently copied, printed, shrunk, and enlarged so that a map with a scale of 1000 feet per inch could range from 950 to 1100 feet per inch or some other scale altogether. The easiest way to compare or check the map scale is to digitize two points on the map with a known distance between them. Dividing the known map distance by the actual distance is the map scale. The two points to use could be a printed map scale, or a mile wide section. Another good way to compute a map is to digitize a known map area as a square mile and back calculate the map scale. This method is especially important on those occasional maps where the y-direction scales is different than the x-direction scale. Once the contours have been properly digitized and their area computed, the reservoir volume can be determined.
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