MJ -- To determine the difference between two directions on a compass the proc below can be used. It can most likely be written shorter, but it should at least give the correct answer.
For two compass courses $a and $b it will calculate the difference in degrees and whether you need to go clockwise (positive return) or counterclockwise (negative return) to go from course $a to $b.
proc anglediff {a b} {
set a [expr {$a%360}]
set b [expr {$b%360}]
set dclockwise [expr (360-$a+$b)%360]
return [expr {$dclockwise<=180?$dclockwise:-(360-$dclockwise)}]
}# and some tests:
proc assert_equal {a b} {
if {$a != $b} { error "assertion $a==$b failed"}
}
assert_equal [anglediff 1 359] -2
assert_equal [anglediff 0 180] 180
assert_equal [anglediff -10 10] 20
assert_equal [anglediff 360 0] 0
assert_equal [anglediff 180 -180] 0
assert_equal [anglediff -10 340] -10