Please watch the videos below for an example workflow of setting up a survey mission, geo-referencing the captured images and uploading to MAPIR Cloud for post processing:
Post-processing survey photos requires only a few steps but there are many different software packages you can use to get the results you desire.
Capture & Apply GPS Locations to Survey Images
First thing you should do when you receive your camera is set the camera’s time to GPS time. GPS time is 17 seconds faster than UTC time. Go to the camera’s settings and use this site to set to GPS time.
Mission planning is based on your aerial platform and what flight controller it is using. The video above shows how to setup a mission with a Pixhawk-based drone and the free Android mission planning application Tower. The flight controller’s log file saved during the mission is used to apply GPS locations to the JPG images, known as geo-referencing the photos. You can watch a video on how to extract the log file from a Pixhawk here using Mission Planner, and this page has links on how to convert the DJI flight logs. Our video above shows how to geo-reference the photos using the converted log file and the program GeoSetter. If you are unable to obtain a log file in a format that GeoSetter can read then we recommend purchasing an external GPS tracker to save your flight path.
Process RAW+JPG Images in Fiji MAPIR Plugin
Once you apply the GPS locations to the JPG images you will want to process the folder of RAW and JPG images with our Fiji MAPIR plugin. This plugin is a pre-processing step to convert the RAW images to TIFFs, copy the EXIF image metadata from the JPGs (including the GPS locations) and correct for the lens vignette. While we recommend only using the processed TIFF images to create your orthomosaic maps (some software will only support the JPGs such as MAPIR Cloud, DroneDeploy, MapsMadeEasy), our plugin will also correct the vignette of the JPGs.
Creating the Ortho-mosaic Images
An ortho-mosaic is a single stitched image containing the many individual photos taken during your survey. Your results will vary though based on the capabilities of the software used to create the ortho image.
Software that is not able to use an image’s GPS data to assist stitching will struggle to create ortho-mosaics of complex subject matter. Complex subject matter are those typically captured when flying over areas where the images captured will look very similar to each other, such as agricultural fields. Software that doesn’t use the GPS data will also have no way to locate the final stitched ortho image onto a map (like Google Earth) because there will be no reference GPS information. We strongly recommend you do not use software that cannot take advantage of the GPS location in the survey images.
There are two options to process your images with GPS information: cloud-based services that do the processing for you or stand-alone programs you run yourself.
Cloud-based packages such as MAPIR Cloud, Drone Deploy, MapsMadeEasy all require you to upload your images to their site and then notify you when the processing is completed. These services vary in how they charge you but the majority have a minimum monthly fee around $100. They also vary in the final results you’ll get from their services, and typically the more expensive “professional” packages provide more outputs with higher cost. Output examples include geo-reference images, DSM maps, KML files and NDVI color-graded images. These cloud-based services often do not support RAW/TIFF files and only work with JPGs, meaning much of the pixel information is changed in the conversion to JPG. They also do not support single band filter cameras for more precise indice/raster calculations, nor reflectance calibration using radiometric ground targetsso you will need to do that using additional software. While these cloud-based sites are adding more and more features each month the best option is to process the images using the following dedicated software packages hosted on your own computer.
Two of the top packages used by our customers are Pix4Dmapper and Agisoft Photoscan. These programs are more commonly known as “point cloud” software. They are called this because they look at each and every point, or more specifically each pixel and match up the images. While they can stitch some image sets this way without GPS information, it will greatly reduce your processing time and increase your success rate if you do use geo-referenced photos. Since these programs create a point cloud they can also output .obj, .mtl and .jpeg texture files to be used in 3D model viewers like p3d and sketchfab. The biggest difference between the cloud-based software and a program like Pix4Dmapper is that Pix4Dmapper allows a user to process multiple cameras in a single project and have each ortho-mosaic share the elevation map file. This means that each of the resulting ortho-mosacis are perfectly aligned with each other and if you brought them in as separate layers to Photoshop each similar pixel would be lined up across the layers. Pix4Dmapper also allows you to calibrate the pixel values and process the orthos into an index image (see the next step below) all without leaving their software package. If you’re looking for the most complete software to post process your images you can’t beat the features that Pix4Dmapper offers. These powerful point cloud software packages typically cost about 3x more than the cloud-based services but the additional cost should be easily amortized over time due to the more accurate, consistent results from properly normalized data.
Create Index Images and Apply Color Map (lut)
Once you have the stitched ortho-mosaics you may want to create a colored gradient image to show the indice (NDVI, ENDVI, GNDVI, OSAVI, RDVI, SAVI, etc.) you’re looking for. The NDVI indice is used to show the general “health” of the survey area and is the most common indice created.
As mentioned above, the cloud processing applications (MAPIR Cloud, DroneDeploy, MapsMadeEasy) are only able to process the Visible Light RGB and NDVI model Survey cameras. The NIR camera can also be used if the service is only processing for DVI with the reflected NIR band in the image.
To process the other single-band camera models (Infrared, Red, Green, Blue) you will need to use a program (Pix4Dmapper) that will create the ortho-mosaics of each camera together with the other cameras. This will align them with each other better than any alignment post-processing application will allow for, and is the only way to process an accurate index image from multiple sensors.
Once you create your index image you should calibrate the pixel values with a ground target of known reflectance properties. Calibration allows you to compare multiple surveys over time and is vital in normalizing your data. Both Pix4D and Fiji allow you to read in target data and calibrate your images.
You can also use what we call a “base calibration” in Fiji to calibrate them at an average reflectance and produce very similar results even without ground targets. We create the calibration files based on sampling our many high quality reflectance targets. This allows any user of our cameras to achieve high quality, detailed survey maps with good calibrated normalized data. This procedure is documented in detail HERE.