© DigitalGlobe 2017
PhotoSat bare ground elevations beneath forest cover are 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.
Wherever there are openings in the tree canopy, PhotoSat measures the tree heights. When there are tree canopy openings about every 100m the PhotoSat estimates of bare ground elevations beneath the tree canopy are usually accurate to about 10% of the tree height. In these conditions, with trees up to 20m tall, the PhotoSat bare ground elevation survey accuracy is usually about 2m.
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.
In areas of temperate climate continuous coniferous tree cover, with very few openings to bare ground, the PhotoSat estimates of bare ground elevations are usually accurate to better than 5m in elevation. In areas of continuous triple canopy tropical forest the PhotoSat bare ground elevations are usually better than 10m in elevation.
LiDAR is the best technology for surveying bare ground beneath forest cover
LiDAR surveying is the best method for surveying bare ground elevations beneath continuous forest cover. Except in cases of extremely dense tropical jungle canopy, a few laser photons almost always penetrate the forest cover and reach the bare ground. While bare ground LiDAR surveys in forested areas are not quite as accurate as LiDAR surveys in areas of exposed bare ground they are much better than all other survey methods.
Combining new PhotoSat surveys with existing LiDAR for cost effective updating of forest covered areas
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. The bare ground elevations of the areas that are still tree covered will not have changed since the LiDAR survey. Areas that have changed since the LiDAR survey will no longer have tree cover and can be updated with the PhotoSat surveying. PhotoSat has incorporated existing LiDAR data for the tree covered areas on many surveying projects
Drone survey bare ground elevations beneath tree canopy are estimates
Like the satellite photos, 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 surveying beneath continuous tree canopy is difficult to impossible
The tree canopy blocks and distorts GPS signals so GPS surveying through forest cover is usually difficult to impossible.