Field Activity 5: Gathering Oblique Images with UAS for 3D model Construction

Introduction:

All of the aerial photography that we have gathered thus far in the class has been taken with the camera pointing directly down at the ground, or nadir. This style of photography is good for producing two dimensional imagery, but doesn't have the depth needed to produce a three dimensional model.  To produce a three dimensional model, the camera must be shifted from nadir to oblique, where oblique camera angle will range between 0 and 90 degrees allowing us to gather images with more depth making it easier to develop a three dimensional model.

Study Area:

For this study, our group traveled to the Eau Claire Soccer Park again in Eau Claire, Wisconsin. The soccer park was a good location because at it's center was a concession stand which was the perfect size to gather enough imagery to create a 3D model with the UAS system that we had. The weather on the day of the study was mostly clear and sunny with very light winds to the south.

Figure 1: Concessions building that was surveyed outlined in red.
North is indicated by red arrow in the bottom right hand corner

Methods:

Figure 2: IRIS multicopter
photo courtesy of drnes.com

This study was done using two UAS systems; first the Iris and then the DJI Phantom. First we used the Iris which was programmed with mission planner to take images at certain intervals during the flight with a non-fisheye lens. The program for the image gathering was designed so that the Iris would be pointing it's camera at the building the entire time while it ascended upwards in a helical pattern from 15 to 26 meters. This program is also featured on the tablet format of mission planner as structure scan mode. Once it reached it's maximum altitude, the Iris then gathered images in a zigzag pattern to gather images of the roof structure. Following this, image gathering ended and the Iris landed.

Figure 3: DJI Phantom Drone
Picture courtesy of www.teamonerepair.com

The second scan of the building was done manually with the DJI Phantom drone. Everyone in the class took turns flying around the building and taking pictures with the camera mounted to the Phantom. The controller had a tablet mounted on it so that the operator could have a first person view of the image before capturing it by pressing a button on the controller. The angle of the camera could be controlled with a wheel on the controller but it mostly stayed at the same angle for the entirety of the scan. There wasn't a strict pattern that was followed when collecting this imagery so some people chose to take some images of the roof, while others to pictures around the perimeter. Images were collected until the Phantom had used two batteries.

Discussion:

This discussion will focus on the image collection, be expecting a future post on the processing of the imagery into a 3D model. Unlike the images that we were used to collecting with the Matrix, the images on the Iris were taken with a Go Pro and therefore were not geo-tagged. This can lead to challenges when trying to pinpoint the location of your 3D model on a software platform like Google Earth. Luckily, however there are other options. The first option you can use is to assign coordinates for your model in Google Earth. A second method you could use to aid in placing your model is a survey GPS. Finally, a program called Geosetter can utilize the telemetry log, match the way-points and time stamp data and infuse this information into the image to give it a set of coordinates. Needless to say, if you are collecting imagery from a device that doesn't geotag images, there are still many ways to do it after the collection.

In the pictures below notice the differences in how the oblique images give the concession building more depth than the nadir image that was taken in a previous study. Even if the roof heights and angles were known, it is also difficult to create a 3D model when you can only see the roof and nothing below it. With the oblique format you can see connections between the roof, walls and pillars along with other building features that are necessary in a detailed model.

Figure 5: Nadir imagery collected in a previous field activity
with the Matrix. Notice how difficult it is to see the angles
and height of the roof  structure and how this would be difficult
to derive a 3D model from

Figure 4: Oblique imagery collected with the Iris multicopter.
Notice how the oblique camera perspective allows for better
perception of depth which is essential to 3D imagery

Conclusion:

This field activity focused on the collection of oblique imagery to be used in the construction of a three dimensional model of a concessions building. Previous imagery collected in the nadir format, though useful for other means is not useful for producing a 3D model as is shown above. With oblique imagery you can image the faces of the building and capture more of the intricate surface details that would be useful in the creation of a 3D model.