Pushbroom Photogrammetric Heights Enhance State-Level Forest Attribute Mapping with Landsat and Environmental Gradients

We demonstrate the potential for pushbroom Digital Aerial Photogrammetry (DAP) to enhance forest modeling (and mapping) over large areas, especially when combined with multitemporal Landsat derivatives. As part of the National Agricultural Imagery Program (NAIP), high resolution (30-60 cm) photogrammetric forest structure measurements can be acquired at low cost (as low as $0.23/km(2) when acquired for entire states), repeatedly (2-3 years), over the entire conterminous USA. Our three objectives for this study are to: (1) characterize agreement between DAP measurements with Landsat and biophysical variables, (2) quantify the separate and combined explanatory power of the three auxiliary data sources for 19 separate forest attributes (e.g., age, biomass, trees per hectare, and down dead woody from 2015 USFS Forest Inventory and Analysis plot measurements in Washington state, USA) and (3) assess local biases in mapped predictions. DAP showed the greatest explanatory power for the widest range of forest attributes, but performance was appreciably improved with the addition of Landsat predictors. Biophysical variables contribute little explanatory power to our models with DAP or Landsat variables present. There is need for further investigation, however, as we observed spatial correlation in the coarse single-year grid (approximate to 1 plot/25,000 ha), which suggests local biases at typical scales of mapped inferences (e.g., county, watershed or stand). DAP, in combination with Landsat, provides an unparalleled opportunity for high-to-medium resolution forest structure measurements and mapping, which makes this auxiliary data source immediately viable to enhance large-scale forest mapping projects.

Automated summary

This study demonstrates the potential of pushbroom Digital Aerial Photogrammetry (DAP) combined with multitemporal Landsat derivatives to enhance forest modeling and mapping over large areas. The National Agricultural Imagery Program (NAIP) provides high resolution photogrammetric forest structure measurements at low cost. DAP shows the greatest explanatory power for a wide range of forest attributes, but performance is improved with the addition of Landsat predictors. Biophysical variables contribute little explanatory power to the models. Further investigation is needed to address local biases.