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Geographical Information Systems (GIS) orthoimagery

Wim Devos

Wim Devos

Outline

• Resolution• Sensor types• Geometry: Orthorectification• Radiometry:

Multispectral/Panchromatic/Pansharpened• Common CAP sensor/platforms

Spec

tral

Res

olut

ion

Topographic Mapping

Transportation

Defense

Urban

AgricultureEarth Resources

Environment

Forestry

100 m 10 m 1 m 0.1 m 0.01 m

Spatial Resolution

Panc

hrom

atic

M

ultis

pect

ral

H

yper

spec

tral

Sensor application segments (2004)

LPIS ?

Airborne Sensors

Airborne vs Spaceborne capture

12 km / 40,000 ft 1.3

million

ft

to

2

.5 m

illion

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

400,0

00 m

t

o

760,0

00 m

OBSOLETE in 2013!

Complementary

Data on demand Can operate in adverse

weather conditions( flying under clouds )

Adaptable resolution0.1 - 0.8 m Pan0.2 - 1.6 m Multispectralby changing flying height

Stereo imagery is inherent

Spaceborne sensors (Hi-Resolution)Airborne digital sensors

Fixed orbit (450-650 km) Availability is weather

dependent Fixed resolution

0.8 m Pan4.0 m Multispectral

Known cost per scene Stereo on demand

12.8m 6.4m. 3.2m 1.6m

0.80m 0.40m 0.20m 0.10m

Resolution (pixel size, ground sampling distance)≡ dimension on the ground

GSD 1.6m

GSD 0.20m

Resolution and detection-interpretation

Size of recognizable objectGSD x 3

Car size ~ 4.5m - 5m GSD 1.6 m x 3 = 4.8 m

Size of interpretable objectGSD x 21

Car size ~ 4.5 - 5m 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 issuesGeometric correction• SRS issues• Inappropriate Ground Control Points• GSD-pixel ratio• DEM weaknessesRadiometry• Inappropriate re-sampling algorithm of GS-DN• Mosaicking• Histogram (saturation, alteration,...)• Processing artefactsGuidance (for LPIS):

http://marswiki.jrc.ec.europa.eu/wikicap/index.php/Orthoimage_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 (H20) NIR reflection• chlorofyl VIS absorption discrimination enhanced

False colour = better contentwater

This tiny peak makes a leaf appear green! Eyes don’t pick up its total NIR reflection/transparency!

false colour G-R-NIR BGR

NIR Red

GreenColor 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 DLR’s 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 criticalthere 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