What is Nadir Imagery?

Nadir imagery is aerial photography that a camera records while it points straight down at the ground. The nadir is the point on the ground vertically below the camera center at the moment of exposure.

What nadir means

A nadir view looks straight down. Remote sensing uses the same word for satellites. Natural Resources Canada calls the ground track directly below a satellite its vertical (nadir) track. ESA calls an instrument that looks vertically down a nadir-viewing instrument.

Searches use many names for the same thing: nadir image, nadir photo, nadir view, nadir perspective and nadir camera. All of them mean a picture from a camera that points straight down.

Nadir, principal point and isocenter

Three points describe where a photograph sits over the ground. The principal point is where the perpendicular through the lens center meets the image. The isocenter lies halfway between the principal point and the nadir.

On a true vertical photograph, all three coincide. No true vertical photograph exists, because every photograph has some tip or tilt. A photograph with less than 3 degrees of tilt counts as vertical for most photo interpretation work.

Nadir angle and off-nadir angle

The off-nadir angle is the angle between the nadir direction and the look direction of the sensor. An angle of 0 degrees means the sensor looks straight down. The angle is different for each pixel of an image.

Satellite operators often point their sensors away from nadir. ESA gives 30 degrees as a typical maximum for optical imagery. The SPOT satellite pointed its sensors up to 27 degrees from nadir. This raised the revisit rate and gave stereoscopic coverage. The swath was 60 km wide at nadir and 60 to 80 km wide when the sensors pointed away.

On a drone, the gimbal sets the angle. In ArduPilot mount control, a negative pitch target points the camera down, and the QGroundControl command Tilt 90 points the mount directly downwards. The gimbal entry explains the axes and modes. TarmacView surface scans fix the gimbal at -90 degrees, which is the nadir position.

Nadir and oblique imagery

An oblique photograph has a tilted camera axis. A photograph with more than 3 degrees of tilt counts as oblique. A high oblique photograph shows the apparent horizon. A low oblique photograph does not.

A nadir image shows the tops of objects that stand directly below the lens. Other objects lean away from the center of the frame, so their sides become visible too. Oblique images show the sides of objects better. A 2021 study of drone images in Sensors found that oblique images give information about building facades that is hard to get from nadir images. In that study the nadir images pointed 90 degrees to the ground and the oblique images pointed 30 degrees from the plumb line. Shadows in oblique images can hide the pixels that the software needs to match.

Relief displacement and building lean

Objects that stand above the ground appear displaced from their true position in a vertical photograph. This is relief displacement. Natural Resources Canada calls it the primary geometric distortion in vertical aerial photographs.

The displacement follows four rules.

• It is zero at the nadir.
• It grows with the distance from the nadir.
• It grows with the height of the object.
• It shrinks as the flying height increases.

The relation is d = r h / H. Here d is the displacement and r is the distance from the nadir to the displaced point. The letter h is the height of the object. The letter H is the flying height above the base of the object. The same relation gives the height of an object from its displacement: h = d H / r. A tall object near the edge of the frame therefore leans away from the center. ESA calls this the building-lean effect and says a lower off-nadir angle reduces it.

Ground sample distance

The ground sample distance (GSD) is the linear dimension of a sample pixel’s footprint on the ground. The ASPRS standard uses it for the collection GSD of a raw, near-vertical image. It computes it from the calibrated focal length and the camera height above the average horizontal terrain.

The GSD is not the same for every pixel. ASPRS says that it varies with terrain height and other factors. The USGS ISIS glossary adds that the simple resolution estimate works well near nadir and loses accuracy quickly as the image looks farther off-nadir. A flight planner such as QGroundControl calculates the ground resolution for the survey altitude and assumes flat ground at the launch altitude. Treat that value as a nominal GSD.

Nadir grid flights

A nadir grid is a grid flight pattern that a drone flies with the camera pointing straight down. QGroundControl builds such a grid over a polygonal area. It lets the pilot set the overlap separately along and across the grid lines.

TarmacView surface scans work this way. They shoot still images along parallel runs with the gimbal at -90 degrees and set the front and side overlap for each scan.

Overlap values depend on the project. A USGS coastal mapping workflow for nadir and near-nadir images gives one example. Its project used about 75 to 80 percent forward overlap and 60 to 70 percent side overlap from 300 m. USGS also warns that too little overlap can cause poor reconstructions and a train-track pattern of gaps. These values come from one project. They are not a rule.

What a nadir-only block can miss

A nadir-only image block has near-parallel viewing directions. James and Robson found in 2014 that this can produce a broad vertical doming error in the terrain model. The cause is a wrong estimate of the radial lens distortion when the software calibrates the camera during processing. They found that oblique images in the network reduce the error.

Roncella and Forlani tested 144 block configurations in a 2021 simulation. Their results show that oblique imaging is a key requisite in flat terrain. The accuracy gap between configurations came close to an order of magnitude. Their introduction cites a recommendation to include oblique images 20 to 45 degrees from nadir, at least 10 percent of the block, to reduce doming. A runway or an apron is close to the flat-terrain case that this finding names.

Accuracy

A nadir image does not guarantee an accurate map. The ASPRS Positional Accuracy Standards judge a product by its root mean square error (RMSE) against check points. Edition 1 (2014) does not tie product accuracy to the GSD of the source imagery, the pixel size or the map scale. Edition 2 (2023) removes the 95 percent confidence level and raises the minimum number of check points from 20 to 30.

Airfield pavement

ASTM D5340 is a visual survey method for the pavement condition index (PCI). FAA AC 150/5380-7B describes it. Federally obligated airports must inspect airfield pavements in detail at least once a year. A PCI survey may extend that interval to three years.

The circular does not mention drones or aerial imagery. It also says that a visual inspection alone cannot show the structural condition of a pavement. A nadir image shows the surface from above and nothing below it. This page makes no claim that a nadir image survey replaces or meets the ASTM D5340 method.