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3) Augmentation Solutions: Another key component for a UAV parcel mapping system, is the incorporation of a GNSS data correction solution needed to improve the accuracy of the raw GNSS observables. UAV survey and mapping companies have typically used RTK GNSS receivers (single base station) for this, which allows for the Rover GNSS data to be corrected to survey grade quality. An example of this is Trimble's Smart GNSS Antenna Target (See Figured 2). The GNSS receiver and ground target combination, can also be used as a check point to confirm the accuracy of the data collect. Another correction methodology is the use of Virtual Reference Stations. An example of this is Continuous Operating Reference Stations (CORS). The data from the CORS can be used for single base station processing or processed in a network solution to produce a moving Virtual Reference Station or VRS that enables high accuracy carrier phase differential GNSS positioning anywhere within the network (the Applanix POSPac SmartBase module is an example of such a solution). The advantage of this approach is that no GNSS reference station hardware is required; however it can take up to 24 hours for the CORS data to be available for data processing, which delays the generation of final mapping results. An alternative technique to establishing a physical reference station or using an existing network of CORS for differential GNSS processing is 'Precise Point Positioning' (PPP). Instead of differencing the rover observables from the reference station observables and cancelling out atmospheric and satellite errors, an advanced model for every aspect of the GNSS error chain is developed and parameterized. The Trimble CenterPoint PP-RTX technology combines the methodology of PPP with advanced ambiguity resolution I N D U S T R Y technology to produce centimeter-level positional accuracies anywhere on the globe. Figure 2: Simple Base Station and Target for Data Corrections. Data & Data Collection and Processing The principle product required for parcel mapping is the orthoimage, which can be generated through photogrammetry using a classic aerial triangulation (AT) process and Ground Control Points (GCP's) for georeferencing, or using the more efficient process referred to as 'Direct Georeferencing'. With AT the UAV will capture a large number of overlapping images, often with an 80% side-lap and an 80% end-lap. This overlap is required such that the same point on the ground is visible in multiple images and from different vantage points, which provides perspective similar to the depth perception the human brain processes via information from both eyes. By using automatic pixel matching methods, and identifying pre-surveyed GCP's in the imagery, the position and orientation (Exterior Orientation or EO) of each photo can be computed using a least squares adjustment (the AT process). Once the EO is computed, orthoimages can be produced and mosaicked to form large image blocks for parcel mapping. The drawback of this method is that a large number of matched pixels and ground control Figure 3: Method of Direct Georeferencing. 0 8 G I S R E S O U R C E S | M A R C H 2 0 2 1

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