Some of the most popular applications for Thermal Imagery include roof inspection, utilities, and water pooling in agricultural fields.Thermal imagery has great potential to provide high value deliverables to your clients. To achieve the full potential of this data there are many parameters to consider through the purchase and operation of your Thermal sensor.
The orthomosaic of Thermal imagery runs through a similar process to our standard orthomosaic with Visual imagery. The software looks for tie points between each image where unique features are found. But with thermal you are looking at temperature changes that create unique features instead of shapes and colors that are seen in your standard visual imagery. This means that there are a slightly different set of variables that you must consider through the collection to obtain a quality dataset.
Choosing a Thermal Sensor
PrecisionMapper is currently compatible with the DJI Zenmuse XT line of thermal sensors. This particular line of sensors has many varitions of specifications to meet users needs. It is important that you choose a variation that will perform best for the stitching process used in PrecisionMapper. Below we have reviewed the optimal camera specifications to use in PrecisionMapper.
Radiometric Thermal- This allows for accurate pixel by pixel readings of temperature in the images. It is a more absolute reading of temperature when compared Non Radiometric models which has a higher accuracy rate and does not allow pixel by pixel readings of temperature.
High Resolution (640 x 512)- The XT sensor comes a few varieties of image dimensions. For best results choose the highest resolution with 640x512 image dimensions.
Focal Length- This specification of the sensor is not as important as the others but is worth mentioning. The XT comes in 6.8 mm, 7.5 mm, 9 mm, 13 mm and 19mm Focal lengths. This will determine the field of view on the imagery and effect how high or low you would need to fly to collect data at a acceptable resolution.
Lenses with lower mm of focal length provide a wider angle view, this can often stretch/distort the image to provide this wide angle.
Lenses with a higher focal length will have less distortion but have a smaller field of view when shooting images. This means that you will have to survey more transects over the area of interest to obtain the overlap that you are looking for.
Recommended Zenmuse XT Sensor Choice- When our PrecisionHawk flight teams survey with Thermal sensors we use the Radiometric version of the Thermal XT with 640 x 512 pixel dimensions and a 13 mm focal length
When collecting your data you are able to use the PrecisionFlight for DJI application. In the application you must plan your flight accordingly to collect quality data for the stitching process in PrecisionMapper. Below we have highlighted a few basic rules behind the flight setup and camera settings that should be followed for a successful stitch in our software.
Overlap- 90% Frontal overlap and 80% side overlap should be programmed in the Advanced Settings of the flight plan
Flight Altitude- This is very dependent on the subject you are flying and the Focal Length of the lens you are using. Our recommendation for the 13 mm focal length is 200 ft altitude or lower.
Flight Speed- Low flight speed- Below 4 meters per second should be set in the Advanced settings of your flight plan. This will prevent any Thermal blur in the source imagery.
Image Format- Tif Images. PrecisionFlight will automatically capture Tif for thermal flights.
Image Type- 14 bit images. PrecisionFlight also chooses the 14 bit variety of capture automatically.
Another important part of flying the Thermal sensor is allowing the it to warm up before operating. You should power the drone with Thermal sensor connected and let it warm up for 5 minutes before you fly your mission. If you have any down time between flights that lasts more than 1 hour the sensor warm up should be done again before flying.
Weather and Environment
Your weather and subject matter in the area of interests can play a big role in how your final product turns out. Below are a few things to look out for when considering the weather and environment of your survey area.
Wind- Any winds higher than 15 mph have been known to change the temperature of the surface. It is important to understand that if you are flying a building the windward facing side of the building is going to be cooled more than the other.
Precipitation- Any form of Precipitation will cool the building and take away the thermal variance in the source data. With no variation of thermal readings in the source imagery the final product may stitch badly or not at all.
Example- After a cool rainy night a parking garage was flown first thing in the morning. Due to the cooling effect of the rain that night there was not enough variation in temperature to provide a quality product.
Humidity- We recommend flying in humidity of 60% or lower. The moisture in the air will cause a haze in your source imagery.
Shadows and Lighting- Shadows and lighting over your area of interest will change the temprature readings. Areas with direct sunlight will always have a higher temperature than the shadows
Solar Reflectance- Any highly reflective objects or subject matter on the ground can cause spikes in the thermal readings and ruin your dataset. To avoid this direct solar reflectance avoid flying at solar noon of your flight location. At this time you have a higher chance of direct reflectance of the sun into the camera lens.
Never point your Thermal sensor at the sun this can ruin your sensor completely