When planning a survey with your drone there are many factors that go into producing a successful map. These must all be considered to come out with the best-looking product. Low overlap or a homogeneous area of interest could cause holes in the map. Flying to low or with insufficient weather might cause bad source image quality which will affect the final stitch.
For further instruction on collecting quality source data with the PrecisionFlight application check out our Capturing Data with the PrecsionFlight App video.
By following the procedures below, you will give yourself the best chance of collecting quality data.
What is an Orthomosaic?
When collecting your imagery in the field you are gathering hundreds or even thousands of images that overlap the field at a consistent rate. The finished “stitched” product of these source images are an orthomosaic.
In the PrecisionMapper software each image is scanned for distinct features and tie points. Within each image thousands of tie points are built between it and other images in the survey. These tie points are found in the regions of overlap that are collected on the field. If there are not enough tie points found in an image it may be thrown out during processing. This will cause holes and coverage issues in your final product.
Flight Altitudes- Grid Flight
Recommendations for flight altitude are related to the Sensor used on your drone. Every sensor has a slightly different field of view and resolution in the source imagery.
By flying to low during a survey you risk blurry images OR images that get left out during the stitching process. By flying at lower altitudes, you are including less distinct features (tie points) within each of the source images. Increasing your flight altitude will always increase the chance a quality final product. Raising flight altitude also allows you to cover larger areas than a low altitude survey.
For more information where to set your Flight altitude for a survey check out our Capturing Data with the PrecisionFlight app Article
Overlap- Grid Flight
Flying with the perfect amount of overlap can produce quality products while using batteries most efficiently. The PrecisionFlight app's default overlap setting is 70% front and side overlap. Frontal overlap tells you how fast the drone will be taking pictures throughout the flight. While side overlap will distinguish the amount of transects a flight will need over the area of interest. Side overlap will have more of an effect on battery life as you increase this value.
For more information where to set your Overlap for a survey in PrecisonFlight check out our Mission Settings article
There are certain situations where increasing your overlap may be necessary.
- Your region is very Homogeneous with few distinct features to be used as tie points (Homogeneous Imagery section of this article)
- You are specifically interested in 3D outputs of your upload (Guidelines of Quality 3D Outputs article)
- You are processing Thermal data in PrecisionMapper (Thermal Imagery article)
Recommended Flight Altitude and Overlap- Grid Flight
|2D Products ||Homogeneous Subject Matter|
|Zenmuse x3||80 m||70% Forward 70% Side||110 m||85% Forward 80% Side|
|Zenmuse x4s||50 m||70% Forward 70% Side||80 m||85% Forward 80% Side|
|Zenmuse x5 15mm||75 m||70% Forward 70% Side||100 m||85% Forward 75% Side|
|Zenmuse x5s 15mm||75 m||70% Forward 70% Side||100 m||85% Forward 75% Side|
|Zenmuse XT||50 m|
90% Forward 90% Side
|Phantom 4 Pro||50 m||70% Forward 70% Side||80 m||85% Forward 80% Side|
|Sequoia||50 m||70% Forward 70% Side||110 m||85% Forward 80% Side|
You should always check the PrecisionFlight live view of the camera before the flight to ensure camera settings are set properly. You can also use the live view of the camera during the survey in PrecisionFlight to check for improper lighting in the sensor.
The image below has over contrast present in the source images. A re flight with different camera settings would be suggested if this is experienced.
When flying an area that is very homogeneous the orthomosaic stitching can be extremely difficult for the processing engines. There are less distinct features across the area which means a higher chance of holes in your final product.
The most common example of homogeneous imagery that we see is Late stage vegetation that is very dense in the field. In this situation you will need to fly altitudes of 100 meters or more (Depending on your sensor) and increase overlap values from the default 70% front and side. For more information these recommendations see the Homogeneous Subject Matter column of the Recommended Flight Altitude and Overlap- Grid Flight table above
To obtain better stitching results over homogeneous features like these consider increasing your Flight Altitude and/or Overlap settings.
Cloud Cover and Time of Day
When it is partly cloudy over your area of interest cloud shadows may move through during the survey. This can cause major changes in lighting throughout the source imagery that will affect stitching. There should be no clouds passing through your area of interest while you are collecting imagery. It is recommended to fly in full sun or full overcast when lighting conditions are most consistent.
To best avoid effects of Vignetting in your source imagery and final products be mindful of the time of day you choose to fly. Vignetting effects imagery most during the solar noon of that particular region (See Solar Vignetting section of this article for more details). Avoid flying from 11 am- 1 pm for best results in your source imagery.
Below is a prime example of orthomosaic that had cloud cover pass over during the collection of the imagery. These changes in lighting will cause inaccurate readings in any algorithms you run on the data.
Vignetting refers to the gradual darkening of an image around the images periphery. This is caused by the position of the sun and how it is reflecting light at the time of the picture.
Solar Vignetting is most intense at the solar noon of a region. At solar noon the suns reflection will be most intense and direct to the camera lens. When a drone is flying taking images looking straight at the ground it will pick up this intensified solar reflectance in its images.Flying at the right time of day can ensure that the suns angle of reflectance is outside of the field of view as the drone is taking images. We recommend avoiding the hours of 11 am- 1 pm for capturing data.