Paulo van Breugel wrote: > I have the following function (to calculate cell area) > > PI=3.1415926536 > RES=0.0008333 > R=6371007 > r.mapcalc "test3 = (sin(y()+$RES/2) - sin(y()-$RES/2)) * ($RES * $PI/180) * > $R^2" > > This will give me a wrong results, apparently because R is an integer > value. If I adapt the formula: > > r.mapcalc "test3 = (sin(y()+$RES/2) - sin(y()-$RES/2)) * ($RES * $PI/180) * > float($R)^2" > > I get the correct result. From the help file I understand that if any of > the arguments is a floating number the result should be a floating number. > But that does not seem the case above so apparently I am missing something > here. Can anybody explain the logic behind the above?
In the first case, 6371007^2 is calculated using integer arithmetic, and the result (40589730194049) will exceed the range of an "int" (2^31-1 = 2147483647), typically resulting in overflow (R^2 will probably equal -2005720447). The value will be converted to "double" for the multiplication, but by that point the error has already occurred. In the second case, R is converted to "float" (single-precision floating-point), and the result of squaring it is well within the range of a "float", although it will be rounded to 40589731037184.0 (using double() instead of float() will use double-precision and avoid the rounding error). -- Glynn Clements <[email protected]> _______________________________________________ grass-dev mailing list [email protected] http://lists.osgeo.org/mailman/listinfo/grass-dev
