Some fuel characteristics, including the depth, loading and bulk density of the forest floor, and proportions of sound and rotten woody material, are not distinguishable in a photograph. If you need these values for your inventory, you will need to measure them or estimate them using a different tool or reference.

Fuels or vegetation may not be visible from a single vantage point at your field site or be obscured by other fuels.  Make sure you make a thorough reconnaissance of the site in making your estimations.

If your site has seasonal variation in fuel conditions (for example, a deciduous forest in leaf or leaf-off condition), be aware of what the photo series data actually represents relative to the current condition of your site.

Large logs (> 3 inches in diameter) are heavy.  Small differences in the number of large logs can translate into substantial differences in woody material loading. 

Many sites will not have an exact match in the photo series.  In these cases, you should include multiple photos in your interpretation.

Bulk density values for calculating litter and duff loading are available in some but not all volumes.

The Natural Fuels Photo Series was developed over many years; earlier volumes do not contain all of the variables collected in later volumes.  In addition, the datasets are somewhat dependent on the ecosystem type.