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

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height estimation of manmade objects using high resolution single look google earth imagery

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

Wing Commander PK Sharma

Joint Director (IMINT)

Indian Air Force

slide2
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
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
contents4
CONTENTS
  • Errors
  • Actual calculations using Digital Globe Data
  • Conclusion
traditional practices
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
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 factor7
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.
slide8

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
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

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

  • Assumptions
  • Date of Pass (DOP)
  • Satellite Image Resolution (IR)
  • Imagery is geometrically corrected

Shadow length estimation

a4

Calculation of building height

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

Zenith

Sun altitude angle

Idealized horizon

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.
shadow profile solar altitude angle
SHADOW PROFILE & SOLAR ALTITUDE ANGLE

SHADOW PROFILE & SOLAR AZIMUTH ANGLE

calculation of sun 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
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 diagrams15
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
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
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

calculation of shadow in dense urban area
Calculation of shadow in dense urban area

Sun

Height H

Height H

αs

HB

HB

HB

αs

Srl

Sla

Slm

Location Latitude & Longitude

Location Latitude and Longitude

Shadow Length Main Building Slm

Shadow Length Adjacent Building Sla

shadow is on downhill

H

θ

β

h``

Δd

Ф

h`` - h` = contour interval Ci

αs

h`

Sib

Sia

Si

Shadow is on downhill

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.

shadow on uphill

H

Δd

Ci

αs

γ

Si

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

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
Errors
  • Resolution of the imaging sensor
  • Dense urban terrain
  • Undulating terrain
observations
Observations
  • Very difficult to ascertain factual unity point coordinate of the shadow
  • imperative that multiple readings are taken and one reading is forecasted
conclusion
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.