Exploring Spatial Information
and Analysis in Agriculture

Advanced Spatial Analysis Issues

Exercise 7 – Comparing Map Surfaces

…pending

Base Maps. The Base Maps needed include:

1997 Yield Map (left) and 1998 Yield Map (right). Both displays use the same theme colors and calculation—Equal Ranges, 7 intervals from minimum to maximum values in the respective maps.

1998 Yield Map with Fixed Theme. This display uses the same interval classes as used for the 1997 Yield map. Note the dramatic differences between the two displays of the 1998 data. The assumptions used in map display and limitations of human analysis makes it very difficult to visually compare maps.

Step 1. …press Grid Analysis button; press Overlay class button; select the Calculate operation.

1997_Yield_Volume - 1998_Yield_Volume.

97-98_Diff_Yield Map. This map shows the absolute difference in bushels per acre from the 1997 and 1998 season. Red indicates areas where the 1998 yield was higher than the 1997 (negative difference values). Green indicates areas where the 1998 yield was lower than the 1997 (positive difference values). The field experienced some hail damage in 1998 that might account for some of the differences in yield.

Step 2. …press Grid Analysis button; press Overlay class button; select the Calculate operation.

(Diff_Yield / 1997_Yield_Volume) *100.

%Diff_Yield Map. This map shows the relative difference as a percent of the 1997 values. Red indicates areas where the 1998 yield was higher than the 1997 (negative difference values). Green indicates areas where the 1998 yield was lower than the 1997 (positive difference values). The field experienced some hail damage in 1998 that might account for some of the differences in yield.