Height Estimation Of Manmade Objects Using High Height Estimation Of Manmade Objects Using High Resolution Single Look Google Earth ImageryResolution Single Look Google Earth Imagery

Wing Commander PK SharmaJoint Director (IMINT)

Indian Air Force

AIMAIM

THE AIM OF THE PAPER IS TO DESCRIBE AN APPROACH FOR ESTIMATION OF HEIGHTS OF MANMADE OBJECTS IN A DENSE

URBAN TERRAIN WITH THE HELP OF SUN AZIMUTH AND ANGLE CALCULATED FROM SHADOW OBSERVED IN IMAGERY OR THE

RECKONING BASED ON THE DATE OF SATELLITE PASS .

CONTENTS

• Shadow FactorShadow Factor

• MethodologyMethodology

• Calculation of Sun altitude angleCalculation of Sun altitude angle

• Calculation of Sun Azimuth angleCalculation of Sun Azimuth angle

• Sun path diagramsSun path diagrams

• Calculation of height of buildingCalculation of height of building– Length measurement Length measurement – Accuracy:Accuracy:– Coordinate and shadow length approximationCoordinate and shadow length approximation– Calculation of shadow length on flat surfaceCalculation of shadow length on flat surface– Calculation of shadow in dense urban areaCalculation of shadow in dense urban area

CONTENTS

• ErrorsErrors

• Actual calculations using Digital Globe DataActual calculations using Digital Globe Data

• ConclusionConclusion

TRADITIONAL PRACTICESTRADITIONAL PRACTICES

• Measurement Of Landforms– Combination of field survey techniques – Analogue and analytical photogrammetry

• Advances in computing power– Digital photogrammetric solutions offers an affordable and cost

effective way of mapping topographic features

• Recent decades– Modern geoinformatic height-finding methods emerged

• Global positioning system (GPS), • Interferometer radar, • Airborne laser scanner (ALS) or LIDAR

SHADOW FACTORSHADOW FACTOR

• Helpful in interpretation

• Provide an idea of the profile

• An idea of the relative height of a OBJECT or OBJECTS which makes identification easier

• Shadows can also reduce or eliminate interpretation in their area of influence, since targets within shadows are much less (or not at all) discernible from their surroundings.

SHADOW FACTORSHADOW FACTOR

• SHADOWS CAST BY ROWS OF TREES IN SPOT IMAGES WERE FIRST USED TO ESTIMATE MEAN HEIGHTS OF TREES

• THE BUILDING HEIGHTS WERE ESTIMATED WITH RELATIVELY HIGH ACCURACY USING SHADOWS IN A SET OF SINGLE-LOOK SPOT PANCHROMATIC AND MULTISPECTRAL IMAGES TAKEN FROM THE SAME SATELLITE.

• THE ACCURACY ACHIEVED WAS BETTER THAN ONE-THIRD THE PIXEL SIZE OF THE SPOT PANCHROMATIC IMAGE.

• Scholars of photogrammetry have been able to extract heights of objects from aerial photographs using parallax in stereo-pair photographs.

• If the sun and sensor geometry are known, it is fairly simple to establish a relationship between shadow lengths and the heights of objects.

• The above usages are however confined to high resolution images, with pixel resolutions much better than the objects being measured.

SHADOW FACTORSHADOW FACTOR

REASON FOR THE SCARCITY OF APPLICATIONSREASON FOR THE SCARCITY OF APPLICATIONS

The resolution of the civilian satellite images is much coarser than that of the aerial photographs and shadows are not well defined for

short and commonly occurring objects. This causes problems in determining

shadow widths that are needed for estimating heights.

MethodologyMethodology

• HEIGHT ESTIMATION IN THE PROPOSED METHODOLOGY IS BASED ON THE SHADOW PROFILING, THEREFORE THE MAIN EMPHASIS IS ON THE EARTH

CELESTIAL MOTION AROUND THE SUN AND THE EFFECTS OF THIS MOTION IN SHADOW FORMATION

Calculation of sun altitude angleCalculation of sun altitude angle(Based on DOP & Location)---a1(Based on DOP & Location)---a1Calculation of sun altitude angleCalculation of sun altitude angle(Based on DOP & Location)---a1(Based on DOP & Location)---a1

Calculation of sun azimuth angleCalculation of sun azimuth angle(Based on DOP & Location)---a2(Based on DOP & Location)---a2

Calculation of sun azimuth angleCalculation of sun azimuth angle(Based on DOP & Location)---a2(Based on DOP & Location)---a2

Actual Shadow length on ImageryActual Shadow length on Imagery(Based on IR & Location)---a3(Based on IR & Location)---a3

Actual Shadow length on ImageryActual Shadow length on Imagery(Based on IR & Location)---a3(Based on IR & Location)---a3

Shadow length estimationShadow length estimationa4a4

Shadow length estimationShadow length estimationa4a4

Calculation of building heightCalculation of building height

AssumptionsAssumptions

•Date of Pass (DOP)Date of Pass (DOP)

•Satellite Image Resolution (IR)Satellite Image Resolution (IR)

•Imagery is geometrically Imagery is geometrically correctedcorrected

AssumptionsAssumptions

•Date of Pass (DOP)Date of Pass (DOP)

•Satellite Image Resolution (IR)Satellite Image Resolution (IR)

•Imagery is geometrically Imagery is geometrically correctedcorrected

Calculation of Sun altitude angleCalculation of Sun altitude angle

• THE SOLAR ALTITUDE ANGLE IS THE ELEVATION ANGLE OF THE SUN. THAT IS, THE ANGLE BETWEEN THE DIRECTION OF THE SUN AND THE (IDEALIZED) HORIZON.

Idealized horizon

Sun altitude angle

Zenith

SHADOW PROFILE & SOLAR ALTITUDE ANGLESHADOW PROFILE & SOLAR ALTITUDE ANGLE

SHADOW PROFILE & SOLAR AZIMUTH ANGLESHADOW PROFILE & SOLAR AZIMUTH ANGLE

Calculation of Sun Azimuth angleCalculation of Sun Azimuth angle

THE SOLAR AZIMUTH ANGLE IS THE AZIMUTH ANGLE OF THE SUN. IT IS MOST OFTEN DEFINED AS THE ANGLE BETWEEN THE LINE FROM THE

OBSERVER TO THE SUN PROJECTED ON THE GROUND AND THE LINE FROM THE OBSERVER DUE NORTH.

Sun Path DiagramsSun Path Diagrams

THE SOLAR ALTITUDE, AND THE SOLAR AZIMUTH, CAN BE READ DIRECTLY FOR ANY THE SOLAR ALTITUDE, AND THE SOLAR AZIMUTH, CAN BE READ DIRECTLY FOR ANY DATE OF THE YEAR AND ANY HOUR OF THE DAY FROM THE SOLAR CHARTS OR SUN DATE OF THE YEAR AND ANY HOUR OF THE DAY FROM THE SOLAR CHARTS OR SUN

PATH DIAGRAMS. PATH DIAGRAMS.

Sun Path DiagramsSun Path Diagrams

What is the method to read the altitude and azimuth angle from the sun path diagram?

– Select the chart of the correct Latitude.

– Select the date line.

– Select the hour line and mark its intersection with the date line.

– Read off from the concentric circles the altitude angle.

– Lay a straight edge from the center of the chart through the marked time point to the perimeter scale and read off the azimuth angle.

Calculation of height of buildingCalculation of height of building

Length measurement • Latitude and Longitude represent the angle portion of a point in space defined

in polar coordinates.

• Using Haversine formula between the points in co ordinate system the length of the shadow is calculated

AccuracySince the earth is not quite a sphere, there are small errors in using spherical

geometry

Location Latitude and

Longitude

Height H

Shadow Length Main

Building Slm

αs

Sun

H = H = SSlmlm(tan (tan αsαs))

WhereWhereαs = solar altitude angleαs = solar altitude angle

The Height of main Building H can be calculated as

Location Latitude

and Longitude

Height H

Shadow Length

Main Building Slm

Shadow Length Adjacent

Building Sla

Sun

Height H

SrlSla

αs

Slm

αsHBHB

Location Latitude & Longitude

HB

H = Sla + [ Slm (tan αs) ]

Where

Sla = Observed Shadow length of Adjacent Building on which the shadow of main building is falling

Slm = Length of Shadow discerned of main building

Αs = solar altitude angle Eq(1.2)

Si

h`

h``

Sib

h`` - h` = contour interval Ci

H

Δdαs

Ф

θβ

Sia

Sib = (sin Ф (Δd))/ sin β

WhereФ = αs – (cos-1(Δd / Ci))

hereФ = angle between slope and solar azimuth

Δd = Shadow length measured in imageryCi = contour height interval β = 180 - αs

The shadow will get elongated in case of downhill Refer Fig the actual length of the shadow Sib will become Δd and

this will cause error in height (H) calculation. The hill slope compensation needs to be undertaken.

Si

Ci

HΔd

αsγ

Sib = (Cos γ) Δd + (Ci / tan αs)

Where

γ = sin-1 ( Ci / Δd)

Shadow will be shortened in case of uphill, Hill compensation can be done using DTM or Map contours, this can be calculated as

Besides these significant shadow estimation errors minor computer error can creep Besides these significant shadow estimation errors minor computer error can creep in the calculation due to computational restrictions of image processing software and in the calculation due to computational restrictions of image processing software and

computer system.computer system.

Errors

• Resolution of the imaging sensor

• Dense urban terrain

• Undulating terrain

N

ObservationsObservations

• Very difficult to ascertain factual unity point coordinate of the shadow

• imperative that multiple readings are taken and one reading is forecasted

SKYSCRAPERSSKYSCRAPERSSHADOW SHADOW LENGTH LENGTH

DEVIATIONDEVIATION

SHADOW SHADOW LENGTH LENGTH

DIFFERENCDIFFERENCE WITH E WITH

CONFIDENCCONFIDENCE = 95% E = 95% (MTRS)(MTRS)

SHADOW SHADOW LENGTH LENGTH (MTRS)(MTRS)

SHANGHAI SHANGHAI WORLD WORLD

FINANCIAL FINANCIAL CENTRECENTRE

0.0090280.00902844

11.7968611.79686418418

388.2787388.27870707

TAIPAI101, TAIPAI101, TAIPEITAIPEI

0.0028150.00281566

3.4901753.490175393393

280.8469280.84690101

ESTIMATEHEIGHT

ACTUALHEIGHT

ACCURACY %

480.51227 492 M 2.334905

473.9875M 508M 5.087687

CONCLUSIONCONCLUSION

• The described method involves usage of sun Path Diagrams and shadow measurements.

• The results of calculations are fairly accurate and reliable.

• Inaccuracies can be corrected mathematically.

• Very useful in case of high resolution satellite data.

Thank U