# Why do Topo / DEM examples not use oblique images?

If you look around the net, most examples of doing topographic mapping for large land areas are only using nadir photos, no obliques. This doesn’t make much sense to me as are the obliques fairly critical in the accuracy of generating Topographics Maps / Digital Elevation Models (DEM) ?  Wouldn’t you get a significantly better result if you flew a nadir (top down) mission then flew the same mission with camera at a 45-degree angle?  Would it be best to fly the obliques at a lower elevation?  Again this is NOT for doing 3D of structures, just land.

Hi Jeff,

Wikipedia defines Orthophotoas…an aerial photograph or image geometrically corrected (“Orthorectified”) such that the scale is uniform; the photo has the same lack of distortion as a map.  Unlike an uncorrected aerial photograph, an orthophotograph can be used to measure true distances, because it is an accurate representation of the Earth’s surface, having been adjusted for topographic relief, lens distortion and camera tilt.

For topographic maps, Pix4D is removing the camera tilt and it is counter productive to use Images with camera tilt.

I didn’t mention Orthophoto in my original post.  An orthophoto is a 2D map.  I was asking about three dimensional work products such as a Digital Elevation Model (DEM).  It would see to me if the site has features with large/steep vertical elements (like steep hills or cliffs), oblique photography would increase the precision of work products showing three dimensional measurements, such as a DEM.

Hi Jeff,

Nadir images are better for generating a more accurate orthomosaic. The orthomosaic generation is based on orthorectification which removes the perspective distortions from the images. The orthorectification process creates an image where every pixel looks like it has been taken directly from above (nadir).

In oblique images, on one side, because the camera’s optical axis is typically pointed at an angle of 45-degrees or more, the effects of building alignment, elevation differences, and non-square pixels are all exaggerated.
On the other side, by taking images from an angle of 45° you can capture more detail on the sides of objects, which improves the quality of the 3D model.
For flat terrain, nadir images taken during a grid flight should be enough for reconstruction. One has to keep in mind that images taken from neighboring flight lines will also capture the “sides” of features, and for land reconstruction, this is sufficient for usual applications.

However, to capture complex 3D structures, it can indeed be interesting to combine both nadir and oblique flights.
Keep in mind that, when working on the images in Pix4D, first process each flight separately, then merge the projects into one (see Merging Projects).

About flight height, check out the following articles:

Regarding elevation. we have two types of output:

• digital surface model DSM: based on the densified point cloud
• digital terrain model DTM: generated by using the point cloud classification to filter out objects to keep only points recognized as terrain. Its resolution is 5*GSD of the project.

Cheers

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