Drone mapping, orthophotoplans, DTM and DEM

Aerial photogrammetry and associated products (orthophotoplans, DTM, 3D models, contour lines, etc.) are now old techniques. We trace the origin of aerial photography to the time of hot air balloons and its rise to the period of the war of 1914-1918.

Requiring an expensive implementation until recently, since requiring the use of expensive cameras with a plane or a helicopter as a vector, it was reserved for the description of large territories. Current techniques allow the production at low cost of orthophotoplans with shots taken by drone or ULM..

In a short time supply has become abundant, but it is diverse. However, an orthophotoplan or a DTM remain real “topographic documents” on which studies and costly work will be based, while topography is a very technical profession which is full of hidden traps (management of the coordinate system, use of the right model geoid, influence of linear weathering, etc.). Because these documents must provide a guarantee in terms of accuracy, because they must be enforceable against third parties, because they must sometimes be justified in front of other professionals, because the third party liability insurance which covers them is specific, it is desirable to use professional surveyors and topographers rather than multi-activity drone operators.

It is our competence, our core business, the summary of our offer.

Extrait

Drone mapping of linear structure

45 km water pipeline

in Namibia

The company ALTEREO responsible for carrying out a preliminary study for the renewal of this piplinee, entrusted us with a mission of drone mapping survey in order to produce an orthophotoplan and a DTM.

The environment was particularly favorable since we were in a desert type landscape with few shrubs as shown in the photo on left.

33 flights and a little more than 3900 photos were necessary to cover the area with at least 5 strips per flight for a restored width of 120 m in the current section plus a few extra widths.
120 GCP’s (Ground Control Point) has been surveyed for georefrencing and quality control.
The longest flight was approximately 2.5 km
The average flight height was 180 m for a GSD of 4.7 cm
Fieldwork was done in less than 6 days, with 2.5 days for GCP’s and 3 days for flights.

For more details on this service, see the online article

Drone mapping for Orthophotoplan and Data Elevation Model (DEM)

Landscaping

in the Normandy countryside.

This magnificent wooded plot, in the middle of which sits a house in the purest Norman style, must be the subject of some landscaping. In order for these to fit in with the logic of the existing, we were asked to produce an orthophotoplan and to determine the height of the surrounding trees.

We carried out this work with a multicopter drone while flying at a height of 50 m to obtain images at 2.5 cm GSD. To georeference the set in the national geodetic system (Lambert 93 and NGF altitudes), we positioned targets that we surveyed with a Trimble GeoXR GPS receiving the differential corrections from the Teria network.

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Using the same images, we were able to create a Digital Elevation Model (DEM)

By knowing the altitude of each point, we can measure the height of the trees (25 meters for those located in the southwest corner).

In another configuration, and with the same technique, we could have determined the isotopes of a pond to develop, draw the contour lines, calculate a volume of earth to move.

Profile extraction

Extraction de profil sous forme graphique

In graphic form

Digital Terrain Model (DTM) - contour lines - profile and cross-section.

Altimetric survey

of an agricultural plot

Opposite, there is an example of a longitudinal profile extraction on the path of the outer wheel of an agricultural pivot. The longitudinal profile (and if necessary the cross sections) can be provided:

In standard formats used by design offices (DXF or other)
As a text file with the abscissas and elevations of the significant points
In graphic form as opposite

Survey of quarries and gravel pits to calculate cubatures.

Much more reassuring than a traditional topographic survey, because it avoids the presence of personnel on foot in areas of evolution of the machines, and just as accurate see more thanks to a representation of the smallest details and ground movements, the calculation of cubatures by aerial photos taken from drone becomes a standard of the profession.

Image processing produces a DTM (Digital Terrain Model) whose resolution depends on the flight height and the focal length of the sensor used. The difference between 2 successive DTMs (flights carried out at different periods) indicates the cubic capacity treated with segmentation by zone if necessary.

In order for this difference calculation to be possible, it is necessary to have placed perennial targets on the ground before the flight to register identical flights. Our skills in GPS, in France and abroad, however allow us to replace destroyed targets, including in areas where the GPRS connection does not allow RTK surveys or in the absence of any geodetic network.

Représentation du MNT par fichier TIFF

Représentation du MNT par courbes de niveau

Ausculation du toit d’un batiment à Paris

Auscultation of the roof of a building in Paris

Chimney stack in close-up view

Detail of a crack

Auscultation of the roof of a building in Paris

Auditing building roofs and chimney stacks is not always easy. The architect in charge of this operation on the building of which we present here some images entrusted to us a mission of shooting, which avoided the employment of a team of rope access technicians.

Working with a drone in Paris cannot be improvised. In terms of air regulations, the flight is classified in scenario S3 within the meaning of l’arrêté du 17 décembre 2015 relatif à l’utilisation de l’espace aérien par les aéronefs qui circulent sans personne à bord and the agglomeration of Paris is classified “P” (prohibited area). In addition, the place is relatively close to Hôpital Lariboisière and in the protection zone of its helicopter service.

To carry out this mission in accordance with the regulations, we had to:

Make a prior declaration to the Paris Prefecture of Police, and obtain confirmation of the non-prohibition by telephone from its Operational Directorate of Technical and Logistics Services one hour before the flight.
Sign a memorandum of understanding with the general aviation service of the DGAC (Civil Aviation Authority) in charge of this sector and contact the control tower of the Paris Heliport just before the flight.

This allowed us to take more than 110 photos from all angles, photos whose resolution (smallest visible detail) is less than half a millimeter. Three are presented below.

Orthophotoplans of a farm and a village in Africa

With aerial partners having light means (airplanes or ULM), we offer the production of orthophotoplans for areas too small for traditional photogrammetry and too large to involve a drone.

The following images were taken as part of a validation test of our production chain, with a ground resolution of 10 cm for the cane field and 5 cm for the village.

Orthophotoplans d'une exploitation agricole et d'un village en Afrique - Toposat

Download the high resolution image in JPEG format (File 28 Mo)

Download the high resolution image in JPEG format (File 11 Mo)