Whenever drops impact on cards or foliage, estimates of their aerodynamic size rely heavily upon the spread factor expression assumed or developed. Uncertainties in spread factor may lead to erroneous estimates of aerodynamic drop size and subsequent calculation of mass and volume of spray material released. Over the last 40 years the USDA Forest Service and its cooperators have used witness cards (because they are inexpensive and easy to use) to collect and interpret spread factors for several hundred pesticide tank mixes, and have conducted extensive tests to quantify the relationship between deposit stain size and in-flight drop diameter. The typical approach has been to generate drops of known size, and observe the resulting stain size after drop settling, impact and drying. This technique first involved magnesium oxide (MgO) coated glass slides, where the size of the impacted drops could be measured by craters left in the coating. These sizes were then compared with stains on paper cards, produced from impaction of the same drop population. Spread factor relationships are known to depend on physical properties of the spray material, state of the drop before impaction, drying conditions, and the type of collector paper. These relationships are expressed mathematically as linear, polynomial, or power law curve fits. A database library of these factors, enabling the extension of existing information to untested conditions, is developed and reviewed in this paper. In place, this database provides an historical record that should benefit future field measurements and data reduction and interpretation.