Requires a Strip Align add-on license for LP360 Drone.

Strip Align on the Sensor tab allows users to perform adjustments to their data to minimize any dynamic trajectory error without leaving the LP360 environment.  Strip Align, also commonly referred to as strip matching, strip adjustment, or line matching, is a common step in lidar workflows.  Its main purpose is to minimize the offsets between different passes or scans of an object, to "match" the point clouds.  It accomplishes this by removing residual systematic bore sight errors that may shift the data pass-to-pass and correcting for any dynamic trajectory errors that may also vary over time.  Matching uses tie points and tie planes from different passes (views) of objects to solve for the optimum parameters that best minimizes the offset in the data. It then uses these parameters to correct the trajectory and regenerate the point cloud, creating a new "aligned" LAS layer.  Since it requires different views of the same tie point/tie plane, it requires overlapping scans of the same target.

LP360's Strip Align tool will also update the EXIF tags of any imagery with the corrected trajectory information (correct the pose information) and re-colorize the point cloud based on the new (optimized) image/point cloud alignment.

When to Use Strip Align

Strip Align is typically run after geocoding but prior to any point classification or value-add processing.  It is an optional step, depending on the degree of mismatch between strips and the final deliverables being generated. However, many users apply it on every project regardless, due to its ease of use.  The main disadvantages to running Strip Align on every project are the extra time to process and extra disk space usage, as a new LAS layer is created.  

There are two main methods you can use to determine if it is necessary to run Strip Align on a given point cloud:

1. Use the Profile View 
   • Draw cross sections in the overlap areas between flight lines, then change the Display in the Profile View to Point Source ID .
   • It is recommended to perform this analysis on flat surfaces; buildings with peaked roofs are especially useful for this visualization as are edges of curbs or vertical walls, but offsets can also be observed on horizontal surfaces such as parking lots.  If no hard surfaces are available in the project area, look at natural slopes or embankments with no ground cover or over-hanging vegetation.
   • Examine the offset between flight lines in the profile.  If there is a significant offset, typically a noticeable shift in the vertical/horizontal feature, then Strip Align should be run.  If all flight lines align well with each other, overlapping each other within the noise envelop of the data, then running Strip Align is not necessary. 
   • In the example below there are three flight lines overlapping on the roof: red, green, and blue.  There is clear separation of the blue line prior to running Strip Align.  After Strip Align all three lines overlap (the blue is now almost directly behind the green).

2. Use the Surface Precision tool.  
   • The Surface Precision tool generates a raster that allows the user to qualitatively see the noise in the point cloud data by rasterizing the standard deviation along the surface normal (SDASN).  Misaligned flight lines will show-up as noisy data in overlap areas in a Surface Precision raster.  
   • Typically, Surface Precision rasters are generated with noise bands from Green (Good) to Red (Bad) based on user-configured thresholds.  In the example below a 2 cm noise band was used to assess the improvement made by strip matching: Green < 2 cm noise (good match) Yellow < 4 cm, Orange <6 cm, and Red > 6 cm noise (bad match).  Actual thresholds will depend on the sensor hardware, as some systems are inherently noisier than others.  Vertical structures, such as trees, show as red both before and after due to random penetration of the lidar pass-to-pass.  In the After raster, the noise in the overlap areas has been reduced to < 2 cm, indicating a good alignment.

How to Use Strip Align

1. Select the Strip Align button on the Sensor tab to bring-up the Strip Align dialog shown above.
2. Select the LAS layer or layers to be adjusted by checking the "Use" box on the appropriate row.
   • Tip: You can select multiple layers, with each layer typically being a single flight.  If you do, the tool will also attempt to correct any misalignment between layers/flights.  However, if one layer has significant error (or bias), it may introduce error into the new aligned LAS layer.  See xxxxx for recommendations on working with multiple layers (or multiple flights/drives/paths).
3. Set the "Fixed" option on layers that should be included in the analysis but not be adjusted. Typically, this is a reference layer or "known good" layer you want to force Strip Align to fit the other layers to.  Selecting Fixed layer(s) will force Strip Align to adjust the non-fixed layers to the reference layers.  This is useful if you have previously adjusted a flight line and debiased it to serve as a "trusted" LAS Layer.  The trusted (Fixed) layer will serve as a "known good" layer when adjusting the other layer(s).
4. Note that certain LAS layers may not have an associated trajectory depending on how they were created. These layers cannot be used for alignment but can be set as Fixed (so other LAS layers will be adjusted to them).

5. Layers not selected as 'Use' or 'Fixed' will be ignored and not used in the Strip Align process.  They will not be corrected/adjusted.
6. There are two modes that can be chosen in Settings when running Strip Align, Large and Small:
   • Large is the default mode and should be used in most circumstances or whenever there are offsets greater than 15 cm between lines.
   • Small can be used, if necessary, when offsets are less than 15 cm and there are many observations to solve (many suitable tie point/tie plane areas for Strip Align to work with).
   • Tip: In general start by running Large and only if this mode does not give acceptable results, re-run using Small.  Note that these two modes are exclusive, you cannot run again on the resulting adjusted LAS layer, you must run each on the original source layer.

7. Having selected the 'Use' and 'Fixed' layers and Large/Small mode, adjust the image-related settings as follows:
   • Update EXIF Tags will overwrite the EXIF tags for each image with new pose information based on the corrected trajectory.  This is optional but recommended.
   • Re-Colorize will reassign RGB values to each point based on the corrected alignment of the images and point cloud after the trajectory correction.  This is optional but recommended.
8. When run, Strip Align will create a new set of corrected LAS files and add them to the project.  To set the Output layer for the corrected LAS data, select either the Create New or the Append options.  Create New will create a new LAS layer; a default new layer name is suggested but can be overwritten if desired.  Or the corrected data can be appended to an existing LAS layer. This Append option is useful if correcting a collection of datasets (multiple LAS Layers) and wanting to compile the results into a single working layer.
9. A Job Manager Identifier tag will be auto assigned for this job based on cycle name (date/time) but can be overwritten.  The Job Identifier helps find the job details in Job Manager when there are a lot of tasks in the queue.
10. Once the Strip Align dialog settings are set as desired, press the Submit button to submit the job to the Job Manager .  A pop-up notification indicating that processing is queued will be invoked.  An email will be sent to the email registered in Project settings when the Strip Align job is queued and when the job completes.
11. Once the Strip Align job dispatches, other operations can be performed in the current project, the project can be closed to work on a different project or LP360 can be closed. Do not log-out of the workstation or shut-down while jobs are running. Any in progress jobs running on the workstation will be lost if it is shut down and there is not a way to restart jobs that are lost in this way. You can lock the workstation without affecting running jobs.  You can open the Job Manager to monitor the status of any job at any time.  
12. An email notification will be sent once the job has completed processing and is ready to be finished. Completing the job requires you to run a manual step in Job Manager.  Open the related project in LP360, if it is not already open, open Job Manager, select the related Ready Strip Align job and then press Complete Job to finish the job and add the new strip aligned LAS layer to your project.  

Additional Licensing Requirements for Strip Align

LP360 Strip Align licensing requires an open HTTPS port (443) to ls65.rlmcloud.com [52.9.236.200] to complete the license validation unless an offline activation has been provided.