Tag Archives: Richmond

Overlapping Photogrammetric systems

View of Point Cloud from side/rear

Spending a lot of time seeing how different photogrammetric systems and point cloud software packages speak to one another. sometimes the interface is elegant, sometime not so much…

Tall Structures and Focal Lengths

The Carillon in Byrd Park - Richmond, VA

The Carillon in Byrd Park – Richmond, VA

I am going to share a quick study I did of the Carillon in Richmond’s Byrd Park and use photographs of this tower structure to demonstrate how different lens focal lengths can work together to access a structure when it is freely accessible (visually) from points of view on grade. In essence, I am trying to answer the question from one of my clients who asks “what kind of resolution can I expect to get for rectified photography of a tall structure (up to 200′ high)?” -and, correspondingly, “what kind of access would be required to obtain these photos?”

So, for starters, the photograph above was shot with a 40mm lens on a full frame DSLR about 250′ back from the face of the tower. It serves to capture the entirety of the structure – but not with particularly high resolution once you “zoom in”, see below:

crop on 40mm shot "zoomed in"

crop on 40mm shot “zoomed in”

Below is a diagram that shows the position of the camera and image capture relative to the tower both in side view and front view.

c40-132

In order to gain sufficient resolution to clearly see the architectural features and construction units with a lens this length on must position the camera a lot closer to the structure – more like 50′ or so away, such as is shown here:

c40-18

This yields a photo that obtains a desirable level of resolution in terms of detail, but only covers a portion of the overall surface. Below I’ll post the “close range”  photo before and after rectification and then I’ll post an additional image that shows something closer to a “100% crop” of the rectified image (since these images are much reduced in size) .

40mm shot ~50' from surface

40mm shot ~50′ from surface

40mm shot ~ 50' from surface - rectified to principal plane

40mm shot ~ 50′ from surface – rectified to principal plane

Detail of Rectified Photo showing a sufficiently rich image resolution to document architectural elements in elevation

Detail of Rectified Photo showing a sufficiently rich image resolution to document architectural elements in elevation

So, in order to cover the fell extent of the tower’s surface, one needs to tak a multiple of images from a variety of points of view and blend them together as a sort of mosaic of rectified images which can be collectively tied together by a measured line drawing.

d118

Shooting overlapping photos from different points of view is easy enough, as we move around the structure laterally… But how do we maintain this level of resolution VERTICALLY? Some answers to this question include gaining higher points of view for additional photographs by using scaffolding, an aerial lift/cherry picker, shooting from an adjacent structure, even a remote controlled helicopter/drone etc… These are each workable solutions – but each carry additional complications and costs. Sometime these types of solutions are required because space around the structure is limited (imagine a tall building on a narrow street in Manhattan). But if a structure is sufficiently free standing, a consistently adequate level of resolution can be obtained by moving away from the building and shooting photos using longer focal lengths. In such cases this can be a very cost effective solution.

The question, then, is how far away do you have to go and what focal lengths need to be used?

Below I’ll post a number of diagrams similar to the one shown above that each include a 200mm expo taken at a variety of distances from the structure.

200mm shot ~400' from structure

200mm shot ~400′ from structure

200mm shot ~200' from structure

200mm shot ~200′ from structure

200mm shot ~75' from structure

200mm shot ~75′ from structure

I tried to shoot the same masonry surface in each case – the center being roughly 150′ or more above grade. In the diagrams’ side views you can see how the amount of foreshortening increases with shots closer to the building. This will be apparent both in the un-rectified shots as well as the rectified shots, shown here:

rectified image from a 200mm shot taken ~75 from the structure

Rectified image from a 200mm shot taken ~75 from the structure

detail of rectified image from a 200mm shot taken ~75 from the structure. Very pronounce foreshortening  and resulting distortion of elements no coplanar with principal place.

Detail of rectified image from a 200mm shot taken ~75 from the structure. Very pronounce foreshortening and resulting distortion of elements no coplanar with principal place.

rectified image from a 200mm shot taken ~200 from the structure

Rectified image from a 200mm shot taken ~200 from the structure

detail rectified image from a 200mm shot taken ~75 from the structure showing much less distortion.

Detail rectified image from a 200mm shot taken ~75 from the structure showing much less distortion of non co-planar elements.

detail from a rectified image from a 200mm shot taken ~400 from the structure. Distortion is reduced even further but distance from the structure starts to have an effect on the image's resolution.

Detail from a rectified image from a 200mm shot taken ~400 from the structure. Distortion is reduced even further but distance from the structure starts to have an effect on the image’s resolution.

In the last image, one can see how as the camera moves away from the building the distortion continues to be mitigated -but the resolution of the image starts to suffer. So, as you move away from the building you have to increase your calibrated focal length in order to maintain a desirable resolution.

Below, I’ll share a number of “slides” that try to summarize what I’ve been trying to describe. Each shows a plan view of the Carillon along with a front and side diagram at the lower corners, with a snap shot of the image captured in the upper left corner. First, two 40mm shots and then a batch of 200mm shots getting progressively  closer to the building.

40mm 50' from building

40mm 50′ from building

40mm 220' from building

40mm 220′ from building

200mm 500' from building

200mm 500′ from building

200mm 400' from building

200mm 400′ from building

200mm 320' from building

200mm 320′ from building

200mm 250' from building

200mm 250′ from building

200mm 200' from building

200mm 200′ from building

200mm 100' from building

200mm 100′ from building

200mm 75' from building

200mm 75′ from building

In fact, this method involves trade offs, as I mentioned in the captions above… I think the most successful arrangement are the shots at about 200′ away – they combine a reasonable amount of “distortion reduction” while maintaining sufficient resolution without having to move too far away from the building. This can be important (not having to go too far from the structure) when shooting tall structures that are adjacent to water or a similar obstacle such as one would finde when shooting LIGHTHOUSES (see below). At 200′ from the building sufficent visual access should be available without the need to rent a boat, even for the bayside of the Cape Lookout Lighthouse.

Cape Hatteras Light

Cape Hatteras Lighthouse

Cape Lookout Lighthouse

Cape Lookout Lighthouse

The Carillon at Byrd Park.

The Carillon at Byrd Park.