Accuracy in Forested Areas

Forest cover interferes to some degree with all types of survey methods.

Accuracy Testing Under Forest

PhotoSat has surveyed several forested test areas where we have bare ground LiDAR survey data. We use these areas to calibrate and improve our measurements and estimates of bare ground elevations in forested areas. We also have projects with several thousand Total Survey points along survey lines beneath tropical forest cover, which we use to improve PhotoSat bare ground surveying.

PhotoSat produces estimates 

To produce bare ground elevation surveys in forested areas, PhotoSat first produces and adjusts a model of the tree canopy height. This tree canopy model is then subtracted from the satellite survey of the top of the tree canopy.

Where openings exist in the tree canopy, PhotoSat measures the tree heights.

For example, when there are openings about every 100 m, PhotoSat estimates of bare ground elevations usually accurate to about 10% of the tree height. For trees up to 20 m tall, PhotoSat bare ground elevation survey accuracy is usually about 2 m.

PhotoSat accuracy depends on the density of the forest:

  • Temperate forest: In areas of temperate climate continuous coniferous tree cover, there are usually very few openings to bare ground. PhotoSat estimates of bare ground elevations are usually accurate to better than 5 m in elevation.
  • Tropical forest: In areas of continuous triple canopy tropical forest, there are few openings at all. PhotoSat bare ground elevations are usually better than 10 m in elevation.

Other Survey Methods

LiDAR surveys can be combined with PhotoSat surveys 

If you already have LiDAR survey data for the forested areas, we can incorporate your existing LiDAR bare ground elevations with new PhotoSat survey data to produce an updated bare ground survey. PhotoSat has incorporated existing LiDAR data for the tree covered areas on many surveying projects.

Even under extremely dense tropical jungle canopy, a few laser photons usually penetrate the forest cover, reaching the bare ground. Unsurprisingly, LiDAR surveys under forested areas are not quite as accurate as LiDAR surveys in areas of exposed bare ground. However, LiDAR generally produces accurate elevation surveys for under forest cover.

Drone surveys depend on the surveyor’s method 

Photos from drones only see the top of the tree canopy. The accuracy of drone survey bare ground elevations beneath tree canopy will depend on the drone surveyor’s method of estimating tree canopy height. Different drone surveyors use different processes and software systems to estimate the height of the tree canopy.

GPS surveys are difficult or impossible 

The tree canopy blocks and distorts GPS signals. GPS surveying through forest cover is usually difficult or even impossible.