Maps Made Easy Drones Made Easy

Overlap Management

Tudor -

Overlap

Consistent overlap between successive images and between passes is the most important driver of how well a three dimensional scene is reconstructed to make a successful map.

The term "image stitching" is frequently used when describing what Maps Made Easy does but it isn't very accurate. Image stitching needs only 20% overlap between images but it doesn't create any elevation or 3D model information. Maps Made Easy's processing is photogrammetric and therefore Map Pilot collects data that is appropriate for this type of processing.

Photogrammetric image processing (the process used to rebuild a scene based on photographs) relies on being provided multiple looks at every object in the survey area. Having these images taken from multiple perspectives allows for the creation of a 3D model. Generally, the more "looks" at an object the system has to work with, the better the outputs will be (within reason).

The absolute bare minimum number of looks that are required to reconstruct a simple object, like a field or pile of dirt, is 4. When the object becomes more complex, like a tree or a building, 9 is the minimum that will required to have a chance at creating a faithful reproduction.

 

Based on the Image Overlap setting in the Map Pilot settings menu, Map Pilot will plan a route and automatically trigger the camera at appropriate times to collect proper data. The timing and positioning of these images is based on the aircraft's altitude and the calculated field of view of the camera.

For more information and help visualizing overlap, check out our article about the overlap reports that get generated by Maps Made Easy: 

http://support.dronesmadeeasy.com/hc/en-us/articles/207148006-Overlap-Reports

 

Altitude Assumption

Being that Map Pilot and the aircraft don't know anything about the objects or terrain in the area being mapped (yet), Map Pilot has to assume the world is flat

If there are tall objects in the survey area or the terrain varies in elevation, the Map Pilot user has to take this into account when planning their flight's cruising altitude and choosing a takeoff location.

 

Terrain

With our altitude assumption, if the terrain is flat the aircraft's actual altitude above ground level (AGL) will be fairly accurate. However if the terrain is higher than the takeoff point, the effective altitude AGL will be less and this will reduce the amount of overlap being collected. If the terrain is lower than the takeoff point, the overlap being collected will be increased.

To deal with this we recommend selecting a takeoff location that will be at, or near, the highest point being surveyed if it is possible. This allows the user to select a lesser overlap setting and the lower terrain will be sure to get plenty of coverage.

In cases where the takeoff point is required to be lower than some terrain a higher overlap setting will be required to ensure enough overlap is achieved on the higher terrain.

NOTE: Due to how Maps Made Easy uses ground reference images to correct for the use of the barometric altimeter in its elevation processing, it is highly recommended to take off from an area of undisturbed ground. Taking off from man-made structures or in areas of heavily modified earth will not correspond well to the reference values we get from the NASA SRTM elevation data and will result in elevation offsets.

 

Camera Triggering

Map Pilot can take images at a maximum rate of 1 image every 2.5 seconds. This is the speed limit of the camera (assuming an appropriate card is used). This limits the speed that can be flow while collecting proper overlap. All speed adjustments and limits are done automatically by Map Pilot. A maximum speed can be selected in the settings, but the maximum speed for the selected altitude and overlap are shown in the Flight Plan statistics pullout panel.

 

 

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Comments

  • Avatar
    Veha La

    Is it possible to use Map Pilot for DJI app to capture images for true 3D modeling? The another app says that it needs 5 different paths to achieve this. "The first path corresponds to the vertical views, while the other fours corresponding to the oblique views with the camera oriented about 45 degrees downward looking in four different directions."

  • Avatar
    Ryan

    We instruct people who are trying to capture oblique images using Map Pilot to manually angle the gimbal up to the desired angle (usually 30-45 degrees) and do a normal flight. You can do another run perpendicular to that if you think it is needed. Doing vertical and oblique is usually not needed and is likely overkill.

    For any complex or overhanging structure manual flight is required to get the low angle shots make it so the overhangs will render well. This cannot easily be automated safely which is why we don't offer it as a feature. Every 3D job is very different and will have varying safety and image collection height requirements for it to turn out well.

    Good luck!

  • Avatar
    Matthew Wilson

    Question of overlap on agricultural ground (corn fields)... I've had a few occurrences where I'm missing large areas of a field after processing. Finally researching a reason, I see that the may not be enough overlap. I'm flying at max altitude (394 feet) and covering roughly 150 acres per flight. After reading the article on missing areas following processing, should I consider a corn field similar to flying over trees? More overlap needed? I'm currently using overlap of 75, should I increase to 85 to get a complete image?

  • Avatar
    Ryan

    80% is USUALLY enough for corn. Corn, especially as it gets taller, represents a challenge to our type of processing since it is almost always in motion on anything but the stillest of days. In the presence of wind it might as well be open ocean. "Amber waves of grain" and all...

    Flying high helps for sure. Move overlap will help too. We have actually added a note in the overlap settings panel that states 80% overlap for corn since this is becoming more common.

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