Obliques for 3D model construction

Introduction:
This activity was our first transition from taking imagery in NADIR format to taking them in oblique format whether that be from high oblique(you cant see the horizon) or low oblique(you can see the horizon). To demonstrate oblique imagery we are going to be producing a 3D representation of a pavilion at the soccer fields. Most of this activity is centered around the data collection and the processing will come at a later date.

Study Area:
Our study area was once again located at the Eau Claire soccer fields across from the universities Bollinger Fields (Figure 1). The actual feature to be mapped was a pavilion located in the middle of the soccer fields (Figure 2). We conducted the study on October 7th at 4pm where there was hardly any winds at all and some cirrus clouds with some wispy icy mares tails on them.



Figure 1: Map showing the soccer fields (Study Area) in relation to the University



Figure 2: Map depicting the pavilion that we took imagery of for our 3D model

Methods:
To introduce us to oblique imagery we took pictures from two separate platforms. The first being the Iris multicopter. The Iris flew in a corkscrew motion so that it took pictures at eye level and then in ascending altitude with the camera angled down towards the building. After the corkscrew was finished at about a height of 26 meters, we did a couple crisscross passes so that we got all of the different angles from the roof. One thing to note is that we were taking these photos with a GoPro. GoPro's do not have GPS associated with their photos which can cause some problems with other image gathering but for this activity it works just fine since the GoPro has such a wide lense. Our professor Joe Hupy made it a point that we take note of the different cameras and their abilities because they all go toward proper misson planning since they all have different uses (Figure 3).


Figure 3:  Professor Hupy explaining the pros and cons of the GoPro

The second mulitcopter we flew was the phantom. We did the same procedure with the Phantom as we did with the Iris. The camera that was on the phantom come standard with the platform and DOES have GPS associated with it. The mission was planned through the Mission Planner software and needed different parameters to be set such as altitude, circle radius, number of turns, and number of corkscrews. This software made the actual flight quite easy to run through, but afterwards we decided we wanted more pictures from eye level so we all got the opportunity to manually fly the phantom (which is quite easy due to its self correction). Some of us flew it via the camera on the IPad (Figure 4) while others walked around the pavilion with the Phantom (Figure 5).

Figure 4: Photo of myself flying the phantom via the IPad

 

Figure 5: Photo of Michael Bomber flying the Iris while Professor Hupy shows where he wants the imagery to be captured
 

Discussion:
This form of data collection is very applicable, especially when a client would want a 3D representation of a feature. Otherwise, you could use oblique imagery to capture a rock face or perhaps a soil profile. I would say that the actual collection of the imagery was a different experience than what we were used to because this time we had the UAS right in front of us the whole time and we knew somewhat what the imagery was going to look like. Also, it would be very difficult to manually take imagery from a NADIR perspective (Figure 6), but taking it manually from an oblique perspective was quite simple (Figure 7 and Figure 8). I would say a similarity between the two collections is that you still want good overlap between your rows of photos to ensure good quality when processing the imagery.

Figure 6: A image taken from NADIR from a previous activity

Figure 7: An image taken at a high oblique angle

Figure 8: An image taken from a low oblique angle

Conclusion:
The is a definite difference between NADIR and Oblique imagery and they both have different uses in the UAS world. One can be good for a large area of interest especially when you are looking at creating a DSM or an orthomosaic. The other is excellent for creating models of vertical structures that do not get much attention from the NADIR collection method since it is taking the photos from straight up, all it would get is the roof or poorly represented snapshot of one side of the structure. With oblique imagery however, you can take advantage of a low flying multicopter that can also gain altitude for some high oblique shots as well and together you can process them into a 3D model which could prove to be very helpful for someone who maybe wants to assess the structural integrity of a building.