A 3D Model of an Existing Building: What It Is Actually For

A 3D model of a building is the deliverable people most often ask about and least often need. It is genuinely impressive to spin on a screen. That is not a reason to pay for one.

There are three reasons that hold up in this part of Arizona. This is about those, about how the model is produced, and about the list of things it cannot do — which is longer than most operators admit and worth reading before you spend $650.

Use one: documenting a building before you renovate it

An architect or designer working on an existing structure needs to know what is there. For a building with no drawings, or with drawings that stopped matching the building decades ago, that normally means someone with a tape and a laser measuring every facade by hand.

A photogrammetric model replaces a good deal of that. It gives the designer measurable elevations of every exterior face, the pitch and extent of every roof plane, parapet heights, the position of openings, and the relationship between the building and its site. It can be exported as a textured mesh or a point cloud and dropped into the design software directly.

It does not replace the designer's own site visit, and for anything structural it does not replace an engineer. It is a measurable record of the outside of the building on the day we flew it.

Use two: a condition baseline for insurance or stewardship

Owners of older commercial buildings in Bisbee, along Allen Street in Tombstone, or on Railroad Avenue in Willcox have a problem that owners of new tilt-up do not: nobody can say with precision what the building looked like last year.

A dated, georeferenced model of the exterior answers that. If a monsoon cell takes part of a parapet off, or a vehicle hits a corner, or a question arises about whether a crack in a brick facade is new, the baseline is a measurable record rather than a memory and a few phone photos. Repeated on a two- or three-year cycle it becomes a record of change, which is the thing an adjuster, a historic preservation board or a prospective buyer actually wants to see.

We are not adjusters and we do not decide claims. We produce the record. For a claim in progress, our inspection documentation is usually the right product rather than a model.

Use three: marketing a facility

A winery, a storage yard, an industrial site or a campus can be shown as a navigable model rather than a set of photographs, and for a buyer or tenant who is not local that is a real advantage. The same orbit flights produce the aerial stills and video as a by-product.

Be honest about the audience. A model of a 1,800 sq ft house sells nothing that twelve good stills do not sell for $175. A model of a 40,000 sq ft processing facility with yard, loading and access is a different proposition.

How the model is made

The method is called structure-from-motion photogrammetry. The aircraft flies several orbits around the structure at increasing height and steepening camera angle — Pix4D's guidance is three passes, with an image every 5 to 10 degrees of arc — so that every point on the exterior appears in many overlapping oblique photographs. Software then searches each image for distinctive feature points, matches those points across images, and from the geometry of the matches works out where every camera was and where every matched point sits in three dimensions. The result is a dense point cloud, which is meshed and textured into the model. The camera positions help but, as the USGS workflow documentation notes, they are not strictly required; the geometry comes from the images themselves.

What it needs from the day

• Consistent light. The software is matching pixels. A shadow that moves across a wall between the first orbit and the third is a different wall to the software. Overcast is ideal; failing that, a tight window with the sun high.
• No wind to speak of. Orbit flights are slow, close and precise. We do not fly them in the afternoon in April.
• Access around the whole structure. A face the aircraft cannot see from several angles is a face that will be missing or distorted.
• Ground control, if any dimension will be stated. Without surveyed control points the model is internally consistent but not anchored — scale and position can drift. If the architect is going to pull a dimension from it, we place and survey control. That is the +$450 line on the rate card, and it is not optional for a stated accuracy.

What it will not do

This is the section to read twice.

• Interiors. The camera is on an aircraft outside. There is no interior in the model.
• Under eaves, overhangs, porches and canopies. Anything the camera cannot see from the air is not reconstructed. Deep overhangs leave a shadowed void where the wall meets the soffit, and the ground under a carport is a hole.
• Glass and reflective surfaces. BYU's civil engineering group, which built a photogrammetric model of their campus, states it plainly: reflective or transparent surfaces produce false points and recorded reflections, and windows, shiny metal and water often do not turn out well. A glass curtain wall models as a dented, wavering surface. Standing-seam metal roofs in direct sun are a lesser version of the same problem.
• Uniform, featureless surfaces. Large smooth white walls and bright flat roofs give the software nothing to match and come back with holes. The same group cites a smooth white canopy as its own worst example.
• Thin things. Antennae, wires, light poles, chain-link, railings. They are either missing or partial.
• Vegetation. A tree against a wall hides the wall and models as a lumpy blob.
• A survey. This is a measurable model of what the camera saw, with accuracy that depends entirely on ground control. It is not a boundary survey, it is not an ALTA survey, and it does not substitute for a licensed surveyor or engineer. Our mapping accuracy article gives the figures and the reasoning.

What it costs

The jump between tiers is flight time and processing time. A single structure is two or three orbit sets and a few hundred images. A facility is multiple flight positions, several thousand images, and a processing run that occupies a workstation overnight. The classified point cloud is for engineers and GIS users who need ground, building and vegetation separated as classes in a LAS file; if that sentence means nothing to you, you do not need it.

Every tier here is over $600, so Zone 2 and Zone 3 travel is waived — a model in Bisbee or Tombstone costs the same as one in Willcox.

Whether you should buy one

If an architect has asked for measurable elevations, if you own an older building and want a dated record of its exterior, or if you are marketing a facility to people who will not visit before they decide — yes, with ground control where any number will be relied on.

If you want to see your house spin on a screen, we will do that too, but we would rather you knew in advance what the $650 does and does not buy. The service page has the deliverable list; send us the address and what the model is for and we will tell you which tier it is and what the flight needs.