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Image Measurement,GSD Calculation, Altitude and Fly Height

GSD Calculation
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Lets start with Figure 1 below. All values represented below is totally based on DJI Phantom 3 Professional drone data.

gsd_value

Figure 1

Hence using formula;

GSD= (Sw x H x 100)/(image width x Fr) = (35mm x 33m x 100)/(4000 x 20)=1.44cm/pixel

The calculation print out from other website http://www.aerogis.de is shown below for validation purposes.

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***GSD CALCULATION REPORT***
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INPUT PARAMETERS:
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Sensor Width in [mm]:..35
Focal Length [mm]:…..20
Image Cols/width:…………4000
Image Rows/height:…………2250
Altitude aGr [m]:……33
CALCULATED PARAMETERS:
—————————-
Sensor Height [mm]:….19.687500000000003
Sensor Pixelsize [mm]:.0.00875
Footprint Width [m]:…57.75
Footprint Height [m]:..32.48437500000001
Footprint Area[sqm]:….1875.9726562500004
Footprint Area[ha]:….0.18759726562500004
Footprint Area[sqkm]:…0.0018759726562500003
GSD in Nadir [m]:……0.014437500000000002
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The GSD calculation results from excel spreadsheet downloaded from https://pix4d.com shown in Figure 2;
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GSD_IMG14
.Figure 2
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Image Measurement 
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The results are identical. We also carried out measurement direct on the image using IC Measure application from https://www.theimagingsource.com to validate that the width of image footprint on the ground which is 58m as calculated previously, see Figure 3. Ground Control Point (GCP) or On Ground Scale (OGS) with diameter of 1.5m was installed for measurement calibration purposes, see Figure 4.
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IC_MEASURE
 Figure 3
 Calibration
 Figure 4
 Altitude and Fly Height
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 The altitude of where the image taken was derived from photo EXIF data, see Figure 5. The EXIF data shows that image was taken at the altitude of 68.717m (hereafter referred as ALP). The original level where drone take off was derived from Google Earth and our 3D reality modelling in Figure 6. The altitude where drone take off is 35.62m (hereafter referred as ALT). Substracting ALP and ALT will get the different of approximately 33m, where also known as “fly height” or “elevation” or H value keyed in Figures 1 and 2. The spatial reference system used to determine altitude in our 3D reality modeling is WGS84:EGM96.
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EXIF_DJI_0014
Figure 5
Coordinate
Figure 6
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Ground Sampling Distance (GSD)-Basic Knowledge Before Flying

Ground Sampling Distance

The Ground Sampling Distance (GSD) is the distance between two consecutive pixel centers measured on the ground. The bigger the value of the image GSD, the lower the spatial resolution of the image and the less visible details. The GSD is related to the flight height: the higher the altitude of the flight, the bigger the GSD value.

Even when flying at a constant height, the images of a project may not have the same GSD. This is due to terrain elevation differences and changes in the angle of the camera while shooting. Since the orthomosaic is created using the 3D point cloud and the camera positions, an average GSD will be computed and used.

Forest and dense vegetation: Flying higher helps for the reconstruction, but the spatial resolution will be lower.

At a defined focal length (for UAV/Drone fixed camera), decreasing the altitude:

  • A smaller area is captured, and therefore:
  • The GSD value will decrease: one pixel will capture a smaller area and therefore the image will have a higher spatial resolution.
  • The image rate needs to be increased to maintain a good overlap.

gsd

 

Picture Overlapping – Recommendation

It is recommended by many people outside there based on their flying experienced where image overlapping ranges from 60% to 90%. We always use 80% for easier setup.

Determining Fly Height

Getting help from the Google Earth is the best choice.

flyheightplanning

 

What is Aerial Photogrammetry

Aerial photogrammetry is the science of obtaining accurate geometric measurements using photographs taken from an aerial platform. It includes aerotriangulation, or block adjustment of multiple photographs using point of common detail normally derived from Real Time Kinematic (RTK) satellite navigation and airborne Global Positioning System (GPS).

aero

Aerotriangulated stereo images is the 3D base product from which a number of 2D and 3D products are derived.

obj

Orthophotos are generated from overlapping vertical aerial photographs and processed to remove perspective and the effect of the terrain. Orthophotos are a fundamental base layer for Geographic Information System (GIS) databases.

Snapshot_2

Planimetric maps, point clouds, contours and features also can be generated from aerotriangulated aerial imagery.

 

General Definition About Aerial Imaging Acquisition and Processing

Aerial photogrammetry:

Aerial photogrammetry is the science of obtaining accurate geometric measurements using photographs taken from an aerial platform. It includes aerotriangulation, or block adjustment of multiple photographs using point of common detail normally derived from Real Time Kinematic (RTK) satellite navigation and airborne Global Positioning System (GPS).

Aerotriangulated stereo images is the 3D base product from which a number of 2D and 3D products are derived.

Orthophotos are generated from overlapping vertical aerial photographs and processed to remove perspective and the effect of the terrain. Orthophotos are a fundamental base layer for Geographic Information System (GIS) databases.

Planimetric maps, point clouds, contours and features also can be generated from aerotriangulated aerial imagery.

 

Aerial acquisition:

Aerial acquisition is the process of planning and coordinating flying missions, maintaining and operating airborne sensors and ensuring that all data is collected and organised for further processing.

 

Aerial survey:

Aerial survey is a general term meaning the collection of information conducted from an airborne platform. Information can be collected from various remote-sensing sensors such as still and video cameras, thermal, multispectral and hyperspectral sensors, LiDAR and even by the best-known sensor i.e. our eyes.

 

Aerial photography:

Aerial photography is the action of taking photographs from an airborne platform such as aircraft, helicopter, kite, blimps or unmanned aerial vehicle. Vertical or oblique photographs can be captured depending on the application intended. In general, aerial photography is mainly used for visualisation and illustration purpose as measurements extracted are not accurate due to distortions.

 

Generating Slope Failure Profile or Cross Sectional from Aerial Image

The fastest way…..2 steps

Software tested : 3Dsurvey

Step 1.

Start with loading all reletad images into the application. Then select “Bundle Adjustment”. Select “Orientate” use “Orientate With Telemetry Data” if you rely on drone picture metadata where no GCP done on site. Proceed with “Reconstruction” and select reconstruction level to “high”, minimum overlapping “2” and select “Optimized Point Cloud”.

01

Step 2

Under “Point cloud” menu select “Calculate New”.

02_cx

Once completed select “Calculate profile” then select “Select Profile Definition Line” then drag a line (where cross-section is require) on the point cloud. Select ‘Profile” tab on the right of your screen. The results of surface profile selected is shown below.

03_cx

This cross-section can be exported to autocad in .dwg format for further process. Note that the profile generated is based on unclasssified point cloud. To generate terrain profile,  point cloud classification process shall be carried out. The contour generated from unclassified point cloud is shown below.

04_cx

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