Serving society Stimulating innovation Supporting legislation Geographical Information Systems (GIS) orthoimagery Wim Devos Wim Devos

Outline Resolution Sensor types Geometry: Orthorectification Radiometry: Multispectral/Panchromatic/Pansharpened Common CAP sensor/platforms

Spectral Resolution Topographic Mapping Transportation Defense Urban Agriculture Earth Resources Environment Forestry 100 m 10 m 1 m 0.1 m 0.01 m Spatial Resolution Panchromatic Multispectral Hyperspectral Sensor application segments (2004) LPIS ?

Airborne Sensors Airborne vs Spaceborne capture 12 km / 40,000 ft 1.3 million ft to 2.5 million ft 1 km / 3,000 ft Spaceborne Sensors 800 km / 500 miles 400 km / 250 miles Spaceborne GSD > 0.80m Airborne Digital GSD 0.20m Airborne Film GSD 0.10m 400,000 m to 760,000 m OBSOLETE in 2013!

Complementary Data on demand Can operate in adverse weather conditions ( flying under clouds ) Adaptable resolution m Pan m Multispectral by changing flying height Stereo imagery is inherent Spaceborne sensors (Hi-Resolution) Airborne digital sensors Fixed orbit ( km) Availability is weather dependent Fixed resolution 0.8 m Pan 4.0 m Multispectral Known cost per scene Stereo on demand

12.8m6.4m.3.2m1.6m 0.80m0.40m0.20m0.10m Resolution (pixel size, ground sampling distance) dimension on the ground

GSD 1.6m GSD 0.20m Resolution and detection-interpretation Size of recognizable object GSD x 3 Car size ~ 4.5m - 5m GSD 1.6 m x 3 = 4.8 m Size of interpretable object GSD x 21 Car size ~ m GSD 0.2 m x 21 = 4.8 m the nature of the object determines the resolution required!

Orthorectification process of removing perspective and terrain distortion using a Digital Elevation Model (DEM) Result = constant scale

In practice raw frame orthorectified frame mosaic

Possible orthorectification issues Geometric correction SRS issues Inappropriate Ground Control Points GSD-pixel ratio DEM weaknesses Radiometry Inappropriate re-sampling algorithm of GS-DN Mosaicking Histogram (saturation, alteration,...) Processing artefacts Guidance (for LPIS): age_technical_specifications_for_the_purpose_of_LPIS age_technical_specifications_for_the_purpose_of_LPIS

example original capture produced orthoimage 50cm GSD turned into 1m pixel

example correct image CRS incorrect image CRS rough DEM smoother result but why? despite looks, vector has true location!

Coordinates are correct at ground level! treetop line – walls (consider the basis) ditches - depressions (consider the top) steep slopes - embankments (take into account)

Why hyperspectral? Atmospheric scattering and absorption The sky is blue because.... Easy sun tan in the mountains because… longer wavelenghts provide a sharper image (contrast ) Reflective behaviour of vegetation turgor (H 2 0) NIR reflection chlorofyl VIS absorption discrimination enhanced False colour = better content water This tiny peak makes a leaf appear green! Eyes dont pick up its total NIR reflection/transparency!

false colour G-R-NIR BGR NIR Red Green Color Composite image in RGB space generated by combining images taken at different wavelengths

Panchromatic band broader band, more light smaller GSD NIR

panchromatic multispectral = bundled product

Pansharpening combines PAN+MS: inappropriate appropriate

Synthetic aperture radars Oblique (side looking) +: 24/7 (active system) - : signal strength = f(geometry, roughness, polarisation, dielectric properties)

C-band fully polarimetric image composed by the dual pol DLRs E-SAR system. The intensity image show the difference on the agricultural fields expressed in different colors. The colors are derived by the combination of different polarisations (red=HH, blue=VV, green=HV) Radar image

Conclusion Ortho-rectification removes perspective and displacement to create a metric canvas suitability of image sources depends on spatial and radiometric properties of the features production by off-the-shelf technology but guidance needs to be followed to prevent loss of information correct interpretation is critical there are many CAPI techniques not discussed here more imagery always provides more information (phenology, changes) INSPIRE annex II theme: free use within public services should be assumed