Head is not horizontal distance. Head is roughly defined as "resistance to flow". Most commonly it is vertical lift. Pool pumps don't usually have much vertical lift, but they do have resistance to flow from pipe, elbows, chlorinators, filters etc..

Here's a pretty good explanation of how to calculate head. http://www.poolspa.com/publications/...5/1/pumps.html

Keep in mind that it is not an exact process.