Activity 5 - Obliques for 3D - Model Construction

Introduction:
So far this semester we have been capturing our images in Nadir or straight down below the aerial device. This works well because the scale of the image remains relatively constant so measuring distances is possible. This week we focused on taking oblique images to process and create a 3D - model. Shooting oblique images means having a tilt of more than 3% and angled compared to nadir where the image is straight vertical down. The oblique images can be stitched together to form an accurate, explorable 3-D model. For this field outing we focused on collecting oblique images to create just that, a precise 3-D model.

Study Area:
This week our study area was the Eau Claire Sports Complex. We specifically focused on the imaging the pavilion found there (Figure 1). This was a familiar location to us but this time we focused on collecting a set of much different images for processing later this semester.

Figure 1 - Pavillion we focused on imaging

The weather during our outing was excellent with it being around 60 degrees, light cloud cover and low winds. There was mare tail clouds in the sky indicative of a storm coming but luckily we avoided any storms that were on their way.

Methods:
The first set of data we collected was done so by the use of the 3DR Iris + accompanied with a GoPro camera. The GoPro is great for capturing video but not ideal for this purpose because there are no geotags associated with each picture taken. This makes it more difficult to make a model out of the images because there is no GPS associated with the images. We will have to match up GPS points from the flight log with the images in order to create a decently accurate model. Using Mission Planner we created a mission plan that is referred to as a "structure scan" this meant the Iris + would autonomously fly itself around the building starting at 5m and gather images every 2 seconds capturing as many angles and views of the structure as possible. Upon doing so we decided that we need to gather images lower than 5m so we did so manually after the autonomous mission was complete. Although our model is not complete yet I will post it when done. Here is a .gif image (Figure 2) of our flight so you can see the number of passes and corkscrews the Iris + did to capture all the angles of the pavilion.

Figure 2 - All the images gathered using the Iris +

The next set of data we collected was using the DJI Phantom 3 Professional. The Phantom was a little more suited for doing this task because unlike the Iris, the Phantom does geotag each image you take making it much easier to create a model with after data processing. With the Phantom all of our images were gathered manually. We each got a chance to fly around the pavilion and gathered pictures every couple of seconds. The .gif image of our flight can be seen below (Figure 3)

Figure 3 - All the images gathered using the Phantom 3

It will be interesting to see the difference between the two models. The Iris was flown almost strictly autonomous with very precise movements and regularly spaced image capturing (every 2 seconds) while the Phantom was manually flown and the images were all taken by the pilot at an irregular time interval. While doing previous field activities we focused on capturing a broad area for the purpose of mapping while with this activity there was emphasis on one area and making sure we were extra thorough in gathering images of one structure.

Discussion / Results:
Before today the only images we had captured of the pavilion looked like this (Figure 4) as you can see our images only focused on the roof. Now using images such as this (Figure 5) we can put together a 3 - D model that focuses on all surfaces and sides of the pavilion. With capturing the roof it really only comes down to needing one image if capturing in nadir but with oblique it takes many passes and varying angled shots to capture the whole roof. Capturing oblique takes a lot more time to cover an entire surface but the level of detail is significantly higher. In previous field outings we've had a strong focus on mapping but with this exercise we weren't necessarily mapping but more so modeling.

Figure 4 - Vertical image taken from Field Activity #3

Figure 5 - One of many oblique images taken of all sides of the pavilion

Conclusion:
Using two different devices we captured the same structure. Using two different methods not only increases our chances at getting a successful model but also shows us there are varying ways to accomplish the same goal. Using GPS data or geotagged images we will be able to create a 3 - D model of a structure using image processing software. The applications for this are really endless, farmers could use the same methods to image their fields, insurance companies for disaster areas, the fire and police services could model crashes and fire scenes. Although capturing in oblique isn't as scale friendly and as easily measurable as nadir it serves a purpose especially when mapping. With nadir there is not as much distortion and oblique can show far more detail such as height. To do modeling using images oblique is necessary yet time consuming.