Use a footing triangulation drawing to locate your footings based off of 2 known points along the house wall. Footing layout plans are available with all of our free deck plans. You can also hire decks.com to create custom deck plans that include a footing layout plan.
Use the dimensions on the footing plan to layout the footings in your yard. This process works best with 2 people with 50' cloth tape measures. Set the tapes at an even height at the points indicated along the house, pull the tapes to match the measurements for each dimension corresponding to the plan. The location where the tapes intersect will be the center of your footing.
Mark the center of the footing with spray paint. You can now verify the locations by measuring from center point to center point. If the footing locations are square you can prepare to dig. If the footing locations appear out of line you should double check that your tapes are level and that the stationary points match the footing layout drawing.