THIESSEN POLYGON AND ISOHYET CONTOUR METHODS FOR MEASURE AND COMPARE THE PRECIPITATION MEAN
A brief practical report
Below is the example of the precipitation mean measurement using thiessen polygon, and isohyet contour methods. Both methods is commonly used in climatology. The result of thiessen polygon and isohyet contour methods shown in figures below :
Tabel 1. Measurement of precipitation depth using Thiessen Polygon
| Station | 24-hours | Total Area | Weighted ppt | |
| (mm) | unit | (%) | (mm) | |
| A | 78 | 0.05 | 0.3 | 24.04438964 |
| B | 115 | 1.7 | 10.5 | 1205.302096 |
| C | 140 | 3.8 | 23.4 | 3279.901356 |
| D | 165 | 2.1 | 12.9 | 2136.251541 |
| E | 150 | 0.95 | 5.9 | 878.5450062 |
| F | 90 | 3.4 | 21.0 | 1886.559803 |
| G | 175 | 1.8 | 11.1 | 1942.046856 |
| H | 155 | 2.42 | 14.9 | 2312.577065 |
| Total | 1068 | 16.22 | 13665.22811 | |
∑ Pn
The average of preciputation depth =
∑ area = 13665.23 / 16.22
= 842.4925 mm
Tabel 2. Measurement of precipitation depth using Isohyet Contour
| Station | Group | Total Area | Weighted ppt | |
| Median | unit | (%) | (mm) | |
| 80 – 90 | 85 | 0.02 | 0.092208391 | 7.837713232 |
| 90 – 100 | 95 | 0.27 | 1.244813278 | 118.2572614 |
| 100 – 110 | 105 | 2.1 | 9.681881051 | 1016.59751 |
| 110 – 120 | 115 | 2.5 | 11.52604887 | 1325.49562 |
| 120 – 130 | 125 | 3.5 | 16.13646842 | 2017.058552 |
| 130 – 140 | 135 | 5.2 | 23.97418165 | 3236.514523 |
| 140 – 150 | 145 | 3.7 | 17.05855233 | 2473.490088 |
| 150 – 160 | 155 | 1.9 | 8.759797142 | 1357.768557 |
| 160 – 170 | 165 | 0.7 | 3.227293684 | 532.5034578 |
| 170 – 180 | 175 | 1.8 | 8.298755187 | 1452.282158 |
| Total | 1300 | 21.69 | 13537.80544 | |
∑ Pn
The average of precipitation depth =
∑ area = 13537.8 / 21.69
= 624.15 mm
CONCLUSION
The result showed the difference between both measurement. This may happen when we use a bigger grid size. The accuracy decrease since the grid cannot cover the area border precisely .
Thiessen Polygon Method is simpler than isohyets contour. Isohyet method is more complex and may lowering the data accuracy. This may happen when the contour are not drawn precisely. Particularly, Isohyets contour method is more subjective than Thiessen Polygon Method.
The difference between two measurement may be caused by inaccuracy when counting the grid.
*Geoinformation for spatial planning class – Batch 6*