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Great Gull Island Unmanned Aircraft System (UAS) Photogrammetric Mapping Project

Owned by the American Museum of Natural History in New York, Great Gull Island is a 17 acre bird

sanctuary located seven miles off shore of Connecticut at the entrance to the “Race” between Plum

Island/Orient Point and Fisher’s Island in New York State. By boat it takes about an hour to reach the

island from Niantic bay in Connecticut.

In 2013 as part of a habitat restoration project funded by the US Fish and Wildlife Service, Connecticut

Sea Grant and the University of Connecticut Extension partnered with Helen Hays to help create a plan

to manage the vegetation with the goal of improved nesting habitat. Helen is the Director of the Great

Gull Island Project. GGI is managed as a Roseate and Common Tern nesting colony.

One portion of the project included creating a high resolution map. The following describes our project

to create the image map of Great Gull Island with Unmanned Aerial Vehicle (UAS) technology. Follow

the links below for details about the process: The preparations required, the flights, and the post

processing Pix4DMapper software.

Flight Operation and Planning:

The unmanned system for this project was a small

electric quadcopter (four prop rotor system) custom

built with Mikrokopter brand flight control and

navigation components. An on board GPS system

provided position control and a gimbaled camera

mount stabilized the camera for vertical photography.

Flying a UAS with the University required applying for

an FAA Certificate of Authorization (COA). We did the

online paperwork and our project was approved (2013-

ESA-16-COA).

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The system is designed to fly autonomously to a pre-programmed set of waypoints and with our

equipment and camera the approximate flight time on a battery was 15 minutes. We planned for less

than 12 minutes for each flight to provide a margin of safety. All required equipment was boxed in a

waterproof case for transport to the island by local fishing boat. We did two trips, covering the West

half April 26, 2013 and the East half May 04, 2013.

Using software from Mikrokopter waypoint flight grids were planned over seven separate sections of

the island. The altitude was set at 40 meters and maintained by a barometric altimeter built into the

flight control system. The camera was triggered manually through a switch on the radio transmitter (not

automated with this design). The plan was to take at least two photographs per waypoint to assure at

least one quality photograph at each point and provide enhanced stereo coverage for later 3D

modelling. Prior to each flight the waypoint grid was uploaded to the UAS from a laptop computer. A

map on the computer allowed tracking the flight in real time and the option to take control if required.

The flight opportunities were limited to the time before the nesting Terns arrived in the May and after

they departed in September. Of the two days we flew our times on the island were limited to about

three hours because wave conditions required early departure by the boat. The seven flights provided

complete coverage of the island with additional off nadir photographs for visuals.

The flight data and control parameters, including coordinates and camera bearing, were recorded and

stored onboard the Mikrokopter as GPX text files using an SD card in the flight controller. These data

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were later downloaded and used for processing the position information for the photographs. Follow

the link below for details.

Processing with Pix4DMapper software:

Pix4DMapper was used to process the UAS photographs into an orthomosaic image and a digital surface

model for use in GIS software. The University acquired a research license to work with the imagery. A

few parameters had to be collected and converted to match the software’s requirements. Once

included the software finished the process, creating a seamless high quality image of the island.

Before running in Pix4D a customized visual basic tool processed the GPX files from the quadcopter,

linked the photographs by time stamp, and output the results into text CSV files. The output column

values were then formatted for use by Pix4D Mapper with the following data fields: image filename,

latitude, longitude, altitude, pitch, roll, horizontal accuracy, and vertical accuracy.

In this project the pitch and roll values were set to zero for processing because the camera was

gimbaled, maintaining a relative nadir value to offset pitch and roll by the quadcopter. Even without a

gimbal the Pix4DMapper adjust for minor differences off nadir. Our output report indicates the majority

of images were within 3.0 and 3.36 degrees of vertical (Omega and Phi).

Horizontal position accuracy was assumed to match the accuracy of the on board GPS unit of about 3 to

5 meters. Vertical accuracy would normally be very high because we used the barometric parameters

not the GPS elevation, however this had to modified to adjust for the starting elevations. Unknown to

us at the time our UAS set and recorded altitudes relative to takeoff location, which in some cases were

10 meters higher than others. This meant while our waypoint flight altitude was set to a consistent 40

meters for each flight those launched from higher locations were affectively up to 50 meters. This was

visibly revealed by initial runs of the Pix4D software by showing the predicted altitude against the

computed altitude from the software.

For the final run of the software all flight altitudes were adjusted by adding LIDAR derived elevation

values for the take-off sites. With these adjustments the altitude still varied between flights, but were

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now linked to a common base. After correcting for the LIDAR derived ground elevation the Pix4D output

showed a close tie between predicted and final altitude values.

The left hand image below shows how Pix4D corrected our original altitudes (blue spheres),

automatically adjusting the values to match what the software calculated from the photo overlap (green

spheres). The right hand image shows how using adjusted takeoff altitudes provided a better match to

the final values determined by the software.

Although Pix4D does its own corrections for our mistakes, when planning future flights the take-off

locations altitude will be added or subtracted from the desired waypoint altitude before flying to create

a consistent flight altitude above the surface. This should improve the results for a given set of flights

and assure proper ground coverage.

Heading was maintained to a single direction throughout each flight within a range of error well handled

by Pix4DMapper. The goal was a heading that aligned the images to True North. When converting

images the quality of the pixel resolution is easier to maintain if less rotation is required during

processing and this orientation makes it easier for the user to quality check the photographs. The April

flight was set approximately 10 to 14 degrees off magnetic North because it was thought an adjustment

was required to account for declination. It turned out the Mikrokopter waypoint manager already takes

declination into account, so the second May flight was set to a heading of 360 degrees. Pix4DMapper

adjusted the images where necessary, showing the readings were within 8.25 degrees of intended

direction (Kappa).

After the flight the photographs were downloaded by USB to a PC computer. The photographs were

organized into two folders, one for each flight day. Each day required a time correction on the camera

to account for time drift in the camera settings. Zero (0) seconds on April 26th, minus two (-2) seconds

for May 5th. To get this time correction an on screen photograph was taken of the official NIST US time

from http://www.time.gov. Taken the same day as the flight, the adjustment is the time difference

between the JPEG file EXIF time and what is visible as captured on screen.

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Over 500 photographs were taken. Each photograph was reviewed in an image viewer. Some of the

images were deliberately taken off nadir after waypoint flights to capture other views of the island,

these and other images not usable for mapping were removed from the mapping project. Images at

extreme orientations and those well below the flight altitude were also removed. Images out of focus or

blurred by motion were removed, however, if there was insufficient overlap by neighboring images to

maintain 3-D vectors at a particular location the photograph was retained. A final count of 445 images

were selected for the mapping portion.

The image file names were run through a Visual Basic program to link the photographs to GPX data from

the quadcopter using the corrected time stamps. Using the output CSV file and a link to the

photographs, the Pix4Dmapper software then evaluated the images and determined 439 images had

enough coverage of inland features to allow for calibration and geolocation.

Ground control points (GCPs) can help Pix4D adjust the final output results to a true location. If a CGP

includes elevation data the software can use the values to lock the approximate altitude to a proper

frame of reference, adjusting for errors in the camera parameters and UAS altitudes. To do this the

software requires at least three GCPs for a 3-D lock. Using online web services eleven (11) NGS survey

points were identified in the records as markers on the island. Of those, three were identified as lost in

the survey report descriptions. Of the eight markers describe as valid five were visible in the aerial

images, providing a good distribution for control across the island.

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No elevation data was included with the NGS ground control descriptions other than relative height of

the objects the markers were place upon. For example in one location a cement pillar has a clear survey

marker embedded in its surface, the report description only provided the height of the pillar relative to

the ground, in this case four feet high. To add elevation records to these points bare earth LIDAR values

were averaged surrounding the marker locations, then the feature elevations from the reports were

added to the result. This technique provided a relatively accurate 3D ground control for the software.

An accuracy range was assigned within the software based on assumptions made for each point.

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After the software was run using available control points the digital surface model showed the West end

of the island “dipped” below the East end by approximately 2 meters. In an attempt to help the

program correct for true elevation at these

locations an artificial control point was created

on a western most feature, the goal was to bring

the West end “up” to a level plane. Because

there were no controls on the far West end of

the island we had to create a GCP for Pix4D to

use. From the first run of the Pix4D software a

clearly visible portion of a raised armament was

selected for use.

While the height of this feature relative to real

world coordinates was unknown, the X, Y

coordinate and the height of the feature relative

to the surrounding ground was considered

accurate within centimeters. With this

assumption these coordinates and relative height

difference were taken from the software and the

height adjusted to real world values by adding an

averaged value from the surrounding LIDAR

pixels. The model was then run again with this

adjusted point included. The result brought the

island height variation within half a meter from

West to East, better matching the water levels

surrounding the island.

The output may not be perfect, but even without true surveyed control points the results proved very

effective, more so when you consider the project data was not originally collected with Pix4D in mind.

In the future 3D survey points could be collected across visible features in the dataset, if the software

was then run again using these points the quality of the output could be improved.

The final product is an ortho mosaic now served online. For ArcGIS users a map service can be brought

directly into your desktop software. For non GIS users there are JavaScript and other ArcGIS online

applications for viewing the data.

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Exported snapshots of the final orthomosaic:

Website links and ArcGIS Service locations:

Great Gull Island: http://greatgullisland.org

American Museum of Natural History: http://www.amnh.org/

CT Sea Grant: http://www.seagrant.uconn.edu/

GGI ArcGIS Map Services:

Ortho: http://clear3.uconn.edu/arcgis/rest/services/Aerials/GGI_UAS2013Ortho/ImageServer

DSM: http://clear3.uconn.edu/arcgis/rest/services/Aerials/GGI_UAS2013DSM/ImageServer

Credits: Project funded by EPA Long Island Sound Study Futures Fund and the National Fish and Wildlife

Foundation, CT Sea Grant and UConn Cooperative Extension, with technical assistance from USFWS,

Univ of Connecticut, MASS Audubon, and Univ of Rhode Island. Don LeRoi of AIS for UAS and flight

support. James Hurd from CAHNR for flight support. Output processed with Pix4DMapper by Pix4D

(non-commercial research license).

Joel Stocker

The University of Connecticut

Cooperative Extension System