To make it an official distance you may want to contact a person to measure out your course to make sure it is the distance you are advertising. The nice part about your usual 5k road race event is that there are so many of them so you can keep improving your time. Participants are looking for consistency in their race courses, so they can see if they improved from their last one. If you have a short course or a course that is too long it can have a largely negative impact on the race participant’s experience. This is similar to having Course Markers to make sure participants know where they are going.
A list of official USA Track and Field course certifiers can be found here:
http://www.usatf.org/events/courses/certification/certifiers.asp
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