Lab 5:  Obliques for 3D-Model Construction

Introduction:

Nadir photography is utilizes single point perspective, meaning that the photo is taken from a completely vertical position. This perspective allows for the scale to remain relatively constant and therefore make measurements easier than working with oblique photographs.  In comparison, oblique photography is characterized by tilting the camera at an angle between the optical axis and the nadir line.  As a result, the scale varies with the angular orientation in addition to the topographic relief.  Oblique images are useful for providing overviews of an area and easier to distinguish and recognize objects.  For this lab exercise, the focus was capturing oblique images for the purpose of processing them into a 3D-model at a later date.  

Study Area:

The study area was the pavilion at the center of the Eau Claire Soccer Park, which is southeast of the intersection of Craig Road and W Hamilton Avenue (Photo 1 &2).  Weather conditions were beautiful, with light scattered clouds and a temperature of 57°F.  Record of the weather conditions may be found in the Field Notes section of this blog. 

Photo 1: Google Maps view showing area of interest at the Eau Claire Soccer park.

Photo 2: Ground view of the pavilion at the Eau Claire Soccer Park.  

Methods:

Both the Iris and the Phantom drones were used during this exercise.  The Iris was programmed using the "Structure Scan" mode on the mission planner tablet app to make circular paths around the pavilion starting at 5m and climbing to 26m at 4m intervals.  The GoPro camera on the Iris was set to the "narrow" lens view, and programmed to take a photo every 2 seconds.  After the Iris completed the mission, the drone was flown in manual mode at a height of approximately 2.5m above the ground to ensure that the entire building was captured by the drone.  

In comparison, the Phantom was used solely in manual mode, and each member of the class was given an opportunity to take photos of the pavilion by circling drone around the building.  Photos were taken using a variety of angles and heights to ensure full coverage.  

Oblique photography data collection for 3D modeling is different than Nadir (mapping) because shadows and the number of angles play a major role in how much detail is captured about the object being modeled.  For that reason, multiple photos of the same spot must be taken at different angles and locations to ensure that the model can be rendered in the proper detail.  In addition, more photos are needed (relative to nadir photography) because oblique photography tends to have high scale variations and radial distortions.

Discussion:

Data for this lab is not yet processed, so discussion will focus on the data collection experience.

During the data collection portion, I found oblique photography to be much more complex because many more variables are involved.  In Nadir photography, the camera angle and height above the ground is fixed, whereas in oblique photography both aspects are altered.  As a result, it may not always be practical to fly pre-planned missions where the camera shutter is set at a specific interval. For example, as seen in photo 2, the pavilion has an overhang that covers part of the lower building.  For this reason, the Iris was flown manually at 2.5m off the ground because it was believed that the original mission flown by the Iris (starting at 5m) was unable to capture the lower part of the building covered by the overhang.  The object chosen for 3D modeling in this lab is relatively simple, however it is easy to see how a more complex structure could require precise angles, and therefore manual mode, to prevent gaps in the data. 

Conclusion:

Oblique aerial photography captures objects at an angle, giving viewers a 3D representation  of an object that is easier to identify than nadir photography.  However, oblique photography should not be used for mapping or measurements because it has a larger degree of distortion that prevent applying the traditional nadir mapping equations.  Full coverage is important in order to obtain a complete rendering of the target object, meaning using mission planner or similar software may not be applicable in situations that require a more delicate touch.  Lastly, oblique photography requires more photos over a smaller areas because variables such as shadows and camera angles are no longer fixed.