Height Estimation Of Manmade Objects Using High Resolution Single Look Google Earth Imagery Wing Commander PK Sharma Joint Director (IMINT) Indian Air Force

AIM 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 Factor Methodology Calculation of Sun altitude angle Calculation of Sun Azimuth angle Sun path diagrams Calculation of height of building Length measurement Accuracy: Coordinate and shadow length approximation Calculation of shadow length on flat surface Calculation of shadow in dense urban area

CONTENTS Errors Actual calculations using Digital Globe Data Conclusion

TRADITIONAL 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 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 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.

SHADOW FACTOR 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.

REASON 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.

Methodology 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 angle (Based on DOP & Location)---a1 Calculation of sun azimuth angle (Based on DOP & Location)---a2 Actual Shadow length on Imagery (Based on IR & Location)---a3 Shadow length estimation a4 Calculation of building height Assumptions Date of Pass (DOP) Satellite Image Resolution (IR) Imagery is geometrically corrected

Calculation 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 ANGLE SHADOW PROFILE & SOLAR AZIMUTH ANGLE

Calculation 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 Diagrams 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 PATH DIAGRAMS.

Sun 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 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 Accuracy Since the earth is not quite a sphere, there are small errors in using spherical geometry

Calculation of shadow length on flat surface Sun Height H αs Location Latitude and Longitude Shadow Length Main Building Slm The Height of main Building H can be calculated as H = Slm(tan αs) Where αs = solar altitude angle

Calculation of shadow in dense urban area Sun Height H Height H αs HB HB HB αs Sla Srl Slm Location Latitude & Longitude Location Latitude and Longitude Shadow Length Adjacent Building Sla Shadow Length Main Building Slm 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)

Shadow is on downhill Si h` h`` Sib h`` - h` = contour interval Ci H Δd αs Ф θ β Sia 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. Sib = (sin Ф (Δd))/ sin β Where Ф = αs – (cos-1(Δd / Ci)) here Ф = angle between slope and solar azimuth Δd = Shadow length measured in imagery Ci = contour height interval β = 180 - αs

Shadow on uphill Shadow will be shortened in case of uphill, Hill compensation can be done using DTM or Map contours, this can be calculated as Si Ci H Δd αs γ Sib = (Cos γ) Δd + (Ci / tan αs) Where γ = sin-1 ( Ci / Δd) Besides these significant shadow estimation errors minor computer error can creep in the calculation due to computational restrictions of image processing software and computer system.

Errors Resolution of the imaging sensor Dense urban terrain Undulating terrain

Actual calculations using Google Earth

Observations Very difficult to ascertain factual unity point coordinate of the shadow imperative that multiple readings are taken and one reading is forecasted SKYSCRAPERS SHADOW LENGTH DEVIATION SHADOW LENGTH DIFFERENCE WITH CONFIDENCE = 95% (MTRS) SHADOW LENGTH (MTRS) SHANGHAI WORLD FINANCIAL CENTRE 0.0090284 11.79686418 388.278707 TAIPAI101, TAIPEI 0.0028156 3.490175393 280.846901 ESTIMATEHEIGHT ACTUAL HEIGHT ACCURACY % 480.51227 492 M 2.334905 473.9875M 508M 5.087687

CONCLUSION 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