Photogrammetric survey what it is and how to get it in 5 simple steps

Photogrammetric surveys allow the collection of spatial and metric data directly from an object by means of image acquisition. Get more info in this insight.

In topography, cartography and architecture, the image acquisition of the surrounding reality is a very important process, subject to continuous technology advancements.

Photogrammetry, in particular, has offered all AEC professionals a myriad of new possibilities. This technique, in fact, allows to carry out surveys way much faster, obtaining a 3D representation of the as-built, called point cloud.

If you are entering the world of photogrammetric survey now, I recommend that you immediately try a free point cloud to BIM software with which you can transform, in a few simple steps, the point clouds obtained from a survey into a BIM model.

Let’s find out now all the peculiarities of a photogrammetric survey.

What is a photogrammetric survey?

Photogrammetric survey is the process that detects an object, a construction or the surrounding environment. The aim is to obtain a three-dimensional model that will be useful for determining shape and extracting measurements, but above all for the generation of a BIM model.

Photogrammetry was born as a technique that allows to acquire the metric characteristics of an object through the acquisition and processing of photographic images. The first applications were performed with stereoscopic frames, that is, two distinct frames that capture the very same object.

The evolution of these techniques has led to the SfM (Structure from Motion) techniques that allows you to obtain a three-dimensional point cloud from multiple photos taken according to precise criteria.

Development of the photogrammetry

A photogrammetry is classified according to the capturing method and the processing types.

Depending on the capturing method, you may have:

• terrestrial photogrammetry: the photographs are taken from the ground, at distances often less than 200 metres, as in the case of building surveys;
• aerial photogrammetry: filming takes place from the sky, often by means of a drone.

Or, depending on the type of processing method, you may find:

• analogical photogrammetry: detected objects are reconstructed with the aid of physical devices;
• analytical photogrammetry: detected objects are reconstructed through calculation tools, the measurements made directly on the frames are processed numerically.

The typical photogrammetric survey outputs: point clouds, topographic maps, processed photographs.

The data collected by photogrammetric surveys allow the following outputs:

• point clouds, identified through their location on a coordinate system (xyz);
• topographic maps, i.e. a highly detailed representation of a territory;
• processed photographs such as straightened images or orthophotoplanes.

How can I create my own photogrammetric survey?

To make a photogrammetric survey you need a digital camera – if it is a terrestrial photogrammetry – or a drone -if it is aerial.

If you want to detect an object or a construction with a camera, it is important to study the object to be detected:

• if you are dealing with a small object, such as a statue, for example, it is advisable to take a series of photos by rotating around it, gradually changing inclination and point of view;
• if it’s a front facade, always remain orthogonal to the plane following an “S” shaped path.

Photogrammetric survey procedure of a facade

Drone photogrammetry is very useful because, thanks to an aerial survey of the site, it allows you to obtain numerous details of every single part of a building:

• detailed and scaled roofing plans;
• very high resolution facade prospect with clear and accurate information;
• three-dimensional models that frame the building around it.

Aerial photogrammetry obtained from a drone survey

How to get a 3D model from a photogrammetric survey: 5 simple steps

To obtain a 3D model from a photogrammetric survey, it is important to follow 5 simple steps:

1. Evaluate the type of subject and take the photos: if it is an architectural detail, it is necessary to define the “route” to be followed with the camera; if it is a larger landscape to be detected with a drone, it is important to define a flight plan, which the pilot will have to perform during the survey phase;
2. Import the photos into the software and extract the data: the photos are automatically straightened and aligned in space. This extracts the “key points” that are necessary to make the so-called image matching, that is to match the photos for the subsequent processing of the point cloud;
3. Create the point cloud: from the key points it is possible to generate a first spare point cloud, which can then be turned into a dense point cloud;
4. Generate the mesh: the point cloud is transformed into a mesh, a continuous surface composed of polygons, whose vertices are the characteristic points of the point cloud itself;
5. Generate the texture: on the mesh that was generated earlier, it is possible to add colors by layering the images from the survey on the mesh (texture mapping) or by transferring the colors from the point cloud to the mesh (color for vertex).

The point cloud can be transformed into a BIM model through the use of a point cloud to BIM software. From the realistic model of the detected object, it is possible to easily obtain sections, elevation views, level curves, renderings, videos. You can immediately test the trial version of a point cloud to BIM software, a really powerful tool that allows you to automatically recognize the different elements present in the model and minimize the possibility of errors.

What are the advantages of using photogrammetric surveys?

Carrying out a photogrammetric survey has many benefits compared to a traditional survey; in particular, there are a number of advantages, such as:

• faster and more accurate data acquisition, detecting entire buildings in half the time;
• quick comparisons between the different data;
• ability to explore the cloud at any time from any digital device through a point cloud viewer;
• three-dimensional viewing of the survey data output;
• isolation of portions of acquired points for analysis and research purposes;
• greater accuracy of collected data;
• ability to create a BIM model easily.