Once you have finished digging your footings you need to measure their depth. When measuring the depth of your hole always hold your tape measure straight up and down or plumb as it is called in the industry, and measure to the top of the hole just below the grass level.
Always take a photo of each footing with a tape measure in the hole, ensuring you can clearly see the bottom of the hole. As you can see in the photo the tape measure is plumb, you can see the bottom of the hole and you can see the numbers on the tape measure. You will also need to take a photo of the width of the hole and it is good practice to make sure that your photos have a date stamp on them.
Why take a photo of your footings? Once you have poured concrete into your footings you are no longer able to see the shape or depth of them. Later on if there is ever a problem with your deck, you have the photos to show what was done with the footings. Also instead of visiting your project, many building certifiers or building inspectors are happy for you to supply photos of your footing holes where you have a tape measure to show the depth and width of the hole. You should always confirm with your building inspector that they are happy to use photos instead of visiting your project