A comparison of hand- and spline-drawn precipitation maps for mountainous Montana

Average annual precipitation for the period 1961-1990 was estimated for a mountainous region in Montana with a Laplacian thin-plate spline (ANUSPLIN) and compared to a hand-drawn map. Input data included latitude, longitude, and elevation from a three-arc-second U.S. Geological Survey Digital Elevation Model of the Bozeman and Billings 1° x 2° topographic quadrangles and precipitation data at 96 stations. The two maps are similar in appearance. Digital comparison of the two maps with ARC/INFO's Grid tools shows that mean annual precipitation for the hand-contoured map is 22.9 inches and for the ANUSPLIN map is 23.7 inches. Of the 5,760,000 cells, 53 percent showed no difference between ANUSPLIN and hand-drawn maps ; 19 percent showed a two-inch difference, and 28 percent showed more than 2 inches difference. Input data and model output at the same location are not different (standard deviation 1.77, p-value 0.76). Hand-drawn maps show two inches more precipitation during the 1961-90 period than during the 1941-1970 period. Similarly, measured data at 73 sites for the period 1961-1990 are on average 2.4 inches higher than the same stations during the 1941-1970 period. The difference is significant (p-value < 0.0001).