The Orthomosaic, or orthophoto, product is strongly correlated with the delivered elevation model and the quality and overlap of the drone images.
Note that in order to create an orthomosaic, the corresponding point cloud and raster DSM will be created in the background as a by-product. Therefore, if the orthomosaic is the required deliverable, the point cloud and raster elevation model can be added to the selection without a significant increase in processing time.
Using the Digital Terrain Model and the Digital Surface Model elevation options, TBC creates a classic orthomosaic.
Using the Digital Surface Model (Highest Quality) elevation option, TBC creates a true orthomosaic, fully automatic.
The classic orthomosaic creation process uses interpolation methods to calculate for each pixel of the captured image the accurate georeferenced position in the ortho.
The DTM does not necessarily need the same raster distance as the final classic orthomosaic. Each orthomosaic pixel determines the related DTM height and will be re-projected with the given camera information to the image frame sensor.
In the image below, the red line represents the site line projected to the DTM. TBC then interpolates the corresponding height from the four surrounding [X,Y,Z] coordinates of the raster (orange circles) and calculates the high resolution orthophoto pixel.
As long as the DTM does not include extreme height errors, the interpolation method of height for each ortho pixel is a very common method for the ortho production and is known as classic orthomosaic generation.
CLASSIC ORTHOMOSAIC CREATION, FROM UAV IMAGES TO DTM TO ORTHO
Classic orthomosaics are typically the main deliverable required for rapid mapping projects such as mining and agricultural application or any open terrain without dense building construction. Objects such as tall buildings or trees can lean in the orthomosaic, but using image-centric selection of all drone images reduces the leaning effect and can result in high quality classic orthomosaic images for the built environment.
ORTHOMOSAIC CREATION PROCESS FOR CLASSIC ORTHOS
Point Cloud Generation Process
Described in previous sections, the point cloud is required to create the orthomosaic. The point clouds can be used for example in open mines and earthwork volume computations.
Ortho Rectification Process
The ortho rectification process is completely embedded in TBC and runs automatically during the orthomosaic creation process. To increase the quality of the final orthomosaics and improve the performance in the orthophoto generation, TBC uses the central part of images (the green area in the graphic below). When the image-centric areas of each image are selected, TBC ortho rectifies each pixel on the ground only one time, which decreases the amount of redundancy for the orthomosaic pixels. This reduces the overall processing time and avoids usage of image content in the furthest parts of the drone image. In this way the quality of the orthomosaic will be improved, as tall buildings “lean” stronger in the corners of the drone image and occlude larger areas. TBC avoids these critical parts of the aerial images and creates orthomosaics with less occluded content, giving the user a more natural orthomosaic product.
This means, users can increase the quality and completeness of the final orthomosaic even more, by flying high overlap in both flight direction and laterally between flight lines.
BENEFITS OF FLYING WITH HIGH OVERLAP FOR THE ORTHOMOSAIC CREATION
Mosaic Generation Process
After rectification of the images, the created ortho images are overlapped with a smaller coverage. The mosaic generation process then stitches the ortho images by sewing them over a seamline along the overlap area of the orthomosaics and combines them into a single mosaic image.
TILING + SEAMLINING OF ORTHOMOSAIC CREATION
For better user experience, TBC creates one single mosaic TIFF tile when the size does not exceed the 4GB limitation of TIFF images. If the final mosaic product is larger, TBC automatically splits the result into multiple mosaic tiles.
Using the Digital Surface Model (Highest Quality), TBC creates a true orthomosaic fully automatic. The true orthomosaic is based solely on the point cloud, without any ortho rectification and seamline generation steps.
TRUE ORTHOMOSAIC CREATION, FROM UAV IMAGES TO DSM (HQ) TO ORTHO
Each final orthomosaic pixel needs a corresponding matched point in the point cloud. To create the true orthomosaic as accurately as possible, the point cloud density should match the ortho image to avoid interpolation and incorrect ortho rectification of single pixels.
True orthomosaics provide the most accurate representation of the visible surface, but the correct height for each pixel can be challenging. Incorrect heights distort the final true orthomosaic image. Another challenge is the occlusion of surface areas. Buildings, trees or other surface objects can block the view to the ground surface, thereby creating an inaccuracy representation in the ortho. True orthomosaics are very useful for dense representations of complex projects. Depending on the level of detail required for building roof edges in city areas, this solution is a strong option in urban environments.
ORTHOMOSAIC CREATION PROCESS FOR TRUE ORTHOS
Transparent background option for orthomosaic creation
Orthomosaics can be created with transparent background pixels for areas and edges that are not covered in the imagery. This option will add an additional color channel to the orthomosaic that contains the transparency information, causing the file size to increase by up to 30%. It is recommended that you select the TIFF with JPEG compression option when selecting the transparent background option to reduce the file size.
CREATE CLEAN EDGE ORTHOMOSAICS WITH THE TRANSPARENT OPTION
NOTE: This article is from a larger workflow bulletin produced by Trimble.