| 261 | | |
| | 261 | If there is a small pitch error, surfaces will be tilted up on one side of the swath and down on the other. In opposing lines, these errors will be on different sides. |
| | 262 | |
| | 263 | * Take opposing flight lines. |
| | 264 | * Find flat surfaces going across the swath (e.g. a road going across-track). |
| | 265 | * Take across-track profiles on the flat surfaces |
| | 266 | * If there is a roll error, the profiles will not align with one another and will have a X shape rather than an overlapping - shape (the flatter the X, the smaller the error) |
| | 267 | |
| | 268 | Vertical displacements due to roll error will be most noticeable at the edges of the swath. For small errors, the nadir point will be roughly the same, for large ones, there will be some across-track shift, but that will be much harder to measure than the vertical errors. |
| | 269 | |
| | 270 | === Heading / Yaw === |
| | 271 | |
| | 272 | If there is a heading error, the left side of swath will move backwards while the right side will move forwards (or vice versa). If you reverse the direction of flight (ie. with opposing flight lines), these errors will overlap (left side down in one direction matches right side up in the other) and not be visible. |
| | 273 | |
| | 274 | * Instead, take two parallel flightlines with 30-50% overlap (more overlap = less chance of detection) |
| | 275 | * Along the mid line of the intersection, find areas where the height of the surface changes in an clear and predictable fashion, with reasonable sampling density (e.g. a road leading up a hill, or rooftops with their peaks going across-track). |
| | 276 | * If there is an along-track shift between features, there is a roll error (size of shift proportional to error) |
| | 277 | * If there is no error, features should overlap |
| | 290 | == Automatic method (using Attune) == |
| | 291 | |
| | 292 | ''Needs per click instructions as it's a bit clunky - I have some of these, but hopefully Mark has more'' |
| | 293 | |
| | 294 | Note that Attune apparently crashes if you use >400MB files, and crashes plenty anyway - save often! |
| | 295 | |
| | 296 | First, classify the LAS files - the automatic process should only be used on ground points (largely to avoid perspective effects on buildings?) |
| | 297 | |
| | 298 | Load the LAS files into Attune and georectify, picking an appropriate pixel size. |
| | 299 | |
| | 300 | 1. Select ~50 - 100 tie points (see below) |
| | 301 | 1. Enter any pre-known/estimate boresight parameters (mirror torsion 100000N) to give it a head start. |
| | 302 | 1. Enter image observation weights (x=0.2, y=0.2, z=0.1m), indicating how accurate we are in each dimension |
| | 303 | 1. Select "use class 2" (ground points), max iterations=50, angular convergence 1x10E-08 |
| | 304 | 1. "Solve" (don't compute torsion parameter) |
| | 305 | 1. Examine aposteriori reference -> if it's ~1-2, stop, otherwise you need to fix bad tie points |
| | 306 | 1. Aim for a standard deviation of ~0.00001 rad (P & R) and ~0.00005 (H) |
| | 307 | 1. To fix bad tie points, sort by the residual error and delete or correct the worst points |
| | 308 | 1. A bad tie point will have a residual error of > 25cm (X & Y) or > 5cm (Z) |
| | 309 | 1. Rerun and try again.. |
| | 310 | |
| | 311 | After completing the Attune process, save the parameters as a reg file (for reference - you will need to manually transfer the numbers to avoid overwriting other parameters). Then check the boresight parameters using the manual procedure above and adjust if required. |
| | 312 | |
| | 313 | === Selecting tie points === |
| | 314 | |
| | 315 | Place tie points on the higher altitude lines only when you're really sure of them - they will always have a greater error due to the pixel size. |