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other depending on accuracy and resolution requirements. Essentially, the challenge is to select the right tool for the job at hand. Large area collects should continue to be serviced through either satellite imagery or manned aerial survey depending on the precision and accuracy required, while small localized data collects are best served with quality UAV survey and mapping systems, which provide both a cost and time savings over other collection methods. Figure 4: Integrated GNSS/INS for Precision Survey and Mapping. points (GCPs) are required to achieve the desired cm level accuracy, and a significant collection overlap for the imagery is needed, each adding time and cost to the data collection and data processing requirement. An alternate and superior approach for photogrammetric georeferencing is the use of Direct Georeferencing (DG). The DG method (see Figure 3) bypasses AT to compute the EO for each image by measuring the position and orientation of an imaging payload directly. This is done to a very high degree of accuracy and precision using survey grade, multi-frequency, multi-constellation Differential GNSS tightly integrated with calibrated inertial sensors. A DG system for UAV's is comprised of a GNSS receiver with a calibrated inertial measurement unit (IMU) – (See Figure 4), along with a survey grade differential correct system (single base station) or virtual reference system and post-processing software. The post-processing software either runs on a desktop or in the Cloud as- as-Service and generates the EO for each image at the time of exposure using the GNSS data, the inertial data, and the augmentation data. Post- processing always produces the highest level of accuracy and fidelity for Direct Georeferencing. I N D U S T R Y Observations and Concluding Comments It should be noted that it is often not a choice between using satellite imagery, or manned, or unmanned imagery for parcel mapping, as these data sets will often complement each Figure 5: Image coverage require without DG vs with DG. The key benefit of using DG instead of AT is productivity. Aerial Triangulation is a labour intensive and time consuming process due to the requirement to collect a network of GCP's and match these up with corresponding features in the imagery. The Direct Georeferencing approach eliminates the need for dense GCP's and significantly reduces the need for a tight network for flight lines as exhibited in Figure 5 below. This means projects can be completed faster and with a lower cost. G I S R E S O U R C E S | M A R C H 2 0 2 1 0 9

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