The gradient is calculated by dividing the change in field value by the distance. You would calculate the difference between the isolines that your line goes to and from. The using an index card, use the little key at the bottom that tells you how much distance each tick mark is on the key and copy it to your index card by putting the index card under the key and drawing a line straight down from the key to the index card. then see the difference between the lines in the distance which is probably either meter, kilometers or miles. Then divide the difference between the difference from the isolines by the actual distance.
