Everything you need to know about topographic surveying with SLAM scanners

These workflows provide reliable ways to survey a site from boundary to boundary. But both present significant challenges.

Here are the most common pain points for traditional surveys.

1. Deliberate and time-consuming
   Surveying with a total station is a meticulous process that requires capturing a lot of measurements to accurately define features. Think of a simple feature like a straight curb: You’ll need to capture a survey point every 15-20 meters to define the line. If the curb includes curves, you’ll need to capture an extra three points. And you will need to capture even more points for more complex features. On a large site with numerous complex features, the number of survey points grows quickly—as does the project duration. And it is not about the number of measurements, but about walking between them. To capture a road, for example, you would walk the length of the curb, trace this feature, walk back down the curb to record the channel line, walk up to the center of the road to record the center line, and finally repeat all this for the other side of the road. So, the length of a simple stretch of road ends up being covered multiple times, taking considerable time and energy.
   
   
2. Perceived slowness with a static scanner 
   Tripod-mounted laser scanners are much faster than traditional total stations, but they can still feel slow and deliberate. Adding the time to set up the scanner, make your capture, and move to the next station, it will take you longer than necessary to get a single point cloud. When a survey requires even a dozen setups, that can mean a long time in the field. 
   
   
3. Traditional surveys are not comprehensive
   Best practice for total station survey says you should only survey features defined in the project scope. This approach, while efficient in terms of project focus, can become inefficient if project requirements change or expand. So the problem comes when a stakeholder contacts you later to ask for survey details on a feature outside the scope, like a boundary or curb line. This requires a return to the site and a second survey, at extra time and expense. 
   
   
4. Gaps in data
   A laser scanner can only capture features within its line of sight. If you don’t plan carefully, accounting for every occlusion and adding extra scans to ensure good coverage, you may end up with blank spots in your scan data. And the feature you’re trying to survey could be lost in one of those blank spots. 
   
   
5. Safety concerns
   The need to physically place prisms or GPS equipment - or detail poles - near survey points can pose significant safety risks. Surveying a road line, for example, means standing in the center of a busy road. It is easy to imagine how this is a serious concern in traditional survey workflows.  
   
   
6. Lack of photographic documentation
   Photographs add rich, valuable context to any survey. Unfortunately, most classic equipment does not automatically capture all the images you want. That means it’s up to you, the surveyor, to manually capture photos. While you’re already busy completing a complex survey. The result is that photographic coverage often takes a back seat. 

Simultaneous localization and mapping (SLAM) scanners promise to change how we perform topographic survey.

A SLAM scanner is much like a terrestrial scanner in that it captures 3D data fit for use in a virtual survey workflow. However, SLAM scanners are different in one very important way: They exploit sophisticated sensors and algorithms to make huge improvements to your 3D capture workflow.

SLAM is not a specific hardware or software but a broad term for a technological process. It was developed in the ‘80s to help robots navigate autonomously through new environments without a map. Today, SLAM is used in various applications, from autonomous vehicles to smartphone augmented reality apps. It is also a crucial element in the latest generation of laser scanners.

There are many approaches to SLAM, but the basic idea is similar across all implementations. To demonstrate how it works, here’s a high-level explanation of the SLAM process in 3D scanning:

1. You turn on the SLAM scanner
2. It captures your surroundings using lidar, cameras, and other sensors
3. The SLAM algorithm processes this data to “identify” features around you
4. It uses these features to build a rough 3D map
5. And estimates its own location on that map
6. When you move the device, it collects new data from its sensors
7. The SLAM algorithm uses this data to improve its 3D map
8. Which helps it improve its location estimate
9. And so on

Repeating these steps thousands of times per second, a top-grade SLAM scanner like NavVis VLX 3 tracks its own path as you carry it. It also builds a crisp 3D map of the site at a very high level of accuracy.

Check out our comprehensive educational guide here for a much more in-depth exploration of SLAM →

You may read that explanation and still think that SLAM feels like a black-box technology. If so, it may help to learn that a good SLAM scanner relies on many of the same principles as a good total-station survey.

Foresight and backsight readings

In traditional total station surveys, foresight and backsight readings are critical for accuracy. Backsight readings involve pointing the total station towards a reference point with a known elevation, while foresight readings are taken towards the new point to be measured. These readings help establish the position and elevation of the total station and the point being surveyed. 

SLAM scanners also reference previously established points to determine their position in three dimensions. While you might make these calculations every so often, a SLAM scanner does it continuously, improving the accuracy and precision of its measurements. 

Traverse closures

When you’re working with total station, you’ll sometimes complete the job by surveying the starting point again. This is called a closed traverse, and it allows you to check and correct for any discrepancies or errors in the measurements. You can do the equivalent with a SLAM scanner by performing a loop closure. 

Though each SLAM scanner offers its own unique workflow, the general idea is the same. Here’s how to capture a topographic survey using NavVis VLX 3:

1. Establish a survey control network across the site
2. Walk the site with a NavVis VLX 3 to capture a 3D data set and HD panoramas
3. Use the built-in touchscreen interface to monitor your scanning progress in real time and mark control points
4. Align the belt hook of NavVis VLX 3 with control points to trigger their capture
5. Save your data and upload it to NavVis IVION for cloud processing and optimization
6. Produce any other desired survey deliverable using specialized software