Doing a job with LiDAR could be more accurate than with conventional topography? If it reduces times, in what percentage? How much does it reduce costs?
Times have definitely changed. I remember when Felipe, a surveyor who did my fieldwork, would arrive with a 25-page notebook of cross sections to generate contour maps. I did not live the time of interpolating on paper but I do remember doing it with AutoCAD without using Softdesk yet. So I interpolated with Excel to know at what distance to place the elevation between the two elevations, and these points were placed on layers of different colors and levels, to finally join them with polylines that I turned into curves.
Although the cabinet work was crazy, it was not compared to the field work that was an art, if you wanted to have enough data to do an acceptable modeling when the altimetry was irregular. Then came SoftDesk, the predecessor of AutoCAD Civil3D that simplified the cabinet and Felipe was in one of my courses learning how to use a total station, which reduced time, increased the volume of points and of course precision.
Stage Drones for civil use breaks new paradigms, under similar logic: Resistance to change in surveying techniques always seeks cost reduction and guarantee of precision. So in this article we will analyze two hypotheses that we have heard there:
Hypothesis 1: Surveying with LiDAR reduces time and costs.
Hpothesis 2: Topography with LiDAR leads to loss of precision.
The experimental case
The magazine ALL systematized a work in which a work was carried out in the data survey of a dike, using the conventional method over 40 kilometers. Separately, in a second work a few days later it was developed using LiDAR topography along 246 kilometers of the same dam. Although the sections were not equal in distance, the equivalent section was equated to make a comparison under similar conditions.
The topographic survey was collected in cross sections every 30 meters, coinciding with existing stations. The transverse points were taken at distances less than 4 meters.
The work was georeferenced with points of the geodetic network, which were validated with geodetic GPS along the axes, and from these the transversal points were surveyed using a combination of virtual reference stations and RTK. It was necessary to take additional points at special slope and shape change sites to ensure consistency of the digital model.
The residual differences between the known points and the coordinates obtained by the GPS were those shown in the table, confirming That conventional lifting is very accurate.
|Maximum Residual||Minimum residual square|
|Horizontal||2.35 centimetres.||1.52 centimetres.|
|Vertical||3.32 centimetres.||1.80 centimetres.|
|Three Dimensional||3.48 centimetres.||2.41 centimetres.|
The LiDAR survey
This was done with an Autonomous Unit flying at a height of 965 meters, with a density of 17.59 points per square meter. They recovered 26 known control points and crossed them against an additional 11 first-order points that were read with geodetic GPS.
With these 37 points the LiDAR data fit was made. Although it was not necessary since the coordinates taken by the UAV that is equipped with a GPS receiver and controlled by base stations, obtained all the time a minimum of 6 visible satellites and a PDOP of less than 3. The distances to the base station were never greater than the 20 kilometers.
A set of 65 additional checkpoints served to validate the accuracy of the LiDAR data. Regarding these points, the following vertical details were obtained:
In urban area: 2.99 cm. (9 marks)
In open field or low grass: 2.99 cm. (38 points)
In forest: 2.50 cm. (3 points)
In bushes or tall grass: 2.99 cm. (6 points)
The image shows the large difference in density between the points taken with LiDAR against the cross sections marked in green triangles.
Differences in Precision
The finding is more than interesting, contrary to the hypothesis that the LiDAR survey does not reach the precision of a conventional survey. The following are RMSE (Root mean square error) values, which is the error parameter between the captured data and the reference checkpoints.
|Conventional topography||LiDAR lifting|
|1.80 centimetres.||1.74 centimetres.|
Differences in Time
If the above has surprised us, see what happened in terms of time reduction in a comparative way between the LiDAR method and the traditional method:
The data collection in the field with LiDAR was only the 8%.
- Cabinet work was only 27%.
- Summing the field + flight + LiDAR cabinet hours against the field data + Conventional topography cabinet, LiDAR required only the 19%.
As a consequence, the 123 hours of work per kilometer of conventional topography were reduced to only 4 hours per kilometer.
In addition, if the total of captured points is divided between the time consumed in the capture and cabinet processes, the conventional method obtained 13.75 points per hour, against 7.7 million points per hour of LiDAR.
Differences in Time
The costs of these modern equipment, with these sensors capturing that amount of points, suggests that the work must be more expensive. But in practice, the reduction of mobilization times and expenses that conventional topography implies, The final cost to the customer of the 246 kilometers resulted with LiDAR 71% lower than the total cost of the 40 kilometers with conventional topography !.
It seems incredible, but the price per linear kilometer with LiDAR was just 12% compared to conventional topography.
Does LiDAR topography totally replace traditional topography? Not in total, since the work with LiDAR always occupies some topography for control points, but it can be concluded that with all the advantages of cost, product quality and time, the work with LiDAR generates results with almost the same precision of the topography conventional.
There will always be pros and cons; the high precision of conventional topography is nostalgic, but the complications of asking for permission to enter private properties, risks of locating in irregular sites, the need for gaps in the face of tall grass and obstacles… it is insane. Of course, the density of forest cover also brings its disadvantages in the case of LiDAR, they are not the same relationship parameters between extremely small projects either.
In conclusion, we are pleased to know how technology has advanced to the degree that for large projects like the one raised, it is necessary to have an open mind and willingness to opt for new and more creative ways of doing topography.