Oblique Images: A View Askew


We have many types of datasets in our toolbelt to view and understand Earth. For instance, satellite imagery and aerial imagery enables us to look down on the earth so we can map it.  Multispectral/hyperspectral imagery is used to observe how land categories change, and lidar allows us to look at features in a 3 dimensional view to quantify change. One of the great benefits of having multiple dataset types is that we can fuse them together to answer bigger, more complex questions. You may think your toolbelt is full, but I encourage you to find room for one more data type: Oblique imagery. This blog post will focus on what oblique imagery is, how it is useful in emergency response efforts, and where you can find it.

Oblique imagery is collected at an off-nadir angle so that you can see and measure the side and top of objects. It gives you a wealth of additional information not typically found in traditional aerial photography that is acquired at nadir, where the camera is pointing straight down from the plane. Notice the differences between the oblique and nadir imagery below. One thing that immediately jumps out is the level of detail along the side of the high-rise building.

ESRI World Imagery (top) and 2015 oblique (bottom) imagery of the MIami, FL area acquired by NOAA’s National Geodetic Survey. Oblique image allows you to see the side of the tall building.
ESRI World Imagery (top) and 2015 oblique (bottom) imagery of the MIami, FL area acquired by NOAA’s National Geodetic Survey.

Oblique imagery is not necessarily a new data type. Folks have been collecting aerial imagery at an angle for over a century. For example, there was German amateur photography pioneer Julius Neubronner who strapped a lightweight camera to a homing pigeon in 1907 to take pictures of surrounding landscapes, or the WW1 pilot who held a camera outside his plane taking pictures at an angle. Older oblique imagery is interesting to look at, and while information can be gleaned from it, these images are really just pretty pictures. 

Oblique aerial imagery acquired from pigeons fitted with cameras in 1907. Includes an image of a pigeon with a camera strapped to its chest.
Oblique aerial imagery acquired from pigeons fitted with cameras in 1907

Today’s oblique imagery is not your (great, great, great) grandfather’s oblique imagery. It’s taken with digital cameras specifically designed for collecting imagery at an angle, and uses GPS and an inertial measurement unit to provide positioning information. As if that weren’t enough, we now have computer software packages that can display these images with their positioning information so that other datasets can be added to the view.

There are a number of reasons why folks may want to use oblique imagery, such as community planning, environmental monitoring, or for situational awareness. However, in my opinion, the best application of oblique imagery is for emergency response support.  

I worked on NOAA’s emergency response efforts after Hurricane Katrina. One of my duties involved answering the public’s questions about flooded houses. It was, at times, heartbreaking to provide the homeowner with an image of their neighborhood. The best information they could glean from the aerial image was whether their house was surrounded by water. They had no way of knowing how much water was in their home, if any at all. Here lies the power of oblique imagery.  It can give us an idea about the impact of flooding and provide us with visual cues to help us deduce the depth of the water. For instance, in the oblique image below it doesn’t look like water made it into some of the houses because we can see that they have been elevated. We can get a feel for the depth of the water by looking at the sequence of utility poles in front of the blue roofed shed. Even more inferences can be made about the depth of the water when we start to compare the oblique image with other sources of ground-based images, such as from Google Street View.

2015 oblique image acquired by NOAA’s National Geodetic Survey to support emergency response efforts along the Edisto River, SC, south of Givhans Ferry State Park.
2015 oblique image acquired by NOAA’s National Geodetic Survey to support emergency response efforts along the Edisto River, SC, south of Givhans Ferry State Park.

There are a number of places on the web that you can go to view or download oblique imagery:  

  • If you are interested in viewing oblique images, then you should head over to Bing Maps or Google Maps.  
  • If you want to look at images acquired after some natural disasters, then check out the Civil Air Patrols oblique imagery on the FEMA GeoPortal
  • If you want to view or download oblique imagery along the coast acquired by NOAA’s National Geodetic Survey, then you should explore NOAA’s Coastal Imagery Viewer. Please note that these images are geoferenced and can be freely downloaded. 

Oblique imagery datasets provide a cornucopia of information that is useful in many situations. I don’t know about you, but I need to look for a belt extender because I need this valuable data type on my toolbelt.

 

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