How Is That Done Again? Changing Specifications

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The changing of specifications continually happens with in our profession. We continually need to adapt to these changes. The trend of changing specifications as LiDAR technology advances has driven most of the profession to the USGS National Geospatial Program Lidar Base Specification. The current specification is the USGS NGP LBS V1.2 (Techniques and Methods 11-B4). The specification for this article will be referred to as the USGS NGP LBS V1.2 specification. This specification also calls for some requirement from The ASPRS Positional Accuracy Standards for Digital Geospatial Data (Edition 1, Version 1.0 November, 2014). The USGS NGP LBS specification has changed a couple of times over the course of the profession and the USGS is in the process of issuing a new specification as the result of new technology such as Geiger Mode LiDAR and Single Photon LiDAR. USGS sees the value in this technology and they want to be able to use this technology for future projects. As a result of the adaptation of this specification over time there has been a need to adopt newer versions of the LAS Format. Since the release and acceptance of the LAS format for LiDAR the LAS format has gone from LAS 1.0 to LAS1.2 and now to LAS1.4. It should be said that LAS1.1 was never really used as quick technological changes made us pretty much skip LAS1.1 for the most part. The USGS also has adopted the Quality Levels (QL) for point spacing and accuracy. This will summarize the changes but for detailed information on all the specifications and how they relate to what is currently required it is recommended to look up all the information on line at the appropriate URLs.

Over the last couple of years FEMA has adopted the USGS NGP LBS completed and this was before the release of the USGS NGP LBS V1.2. The good thing about this is that most all the LiDAR being created now will meet the USGS specifications. Additionally, it should be noted most all LIDAR being collected for any governmental agency is following this specification whether it is State, Local, or federal agencies procuring the LiDAR. This specification was released in November of 2014 so it has been around for some time. The USGS needed to transition from version 1.1 to version 1.2 so the time it took to actually see new data available was probably sometime in 2015 because the projects being conducted in early to middle 2015 probably still called the older specifications. It is difficult when a specification is changed for the procurement process because work needs to be done but specifications need also to change to provide the best data.

The changes form version 1.1 to 1.2 are well known since the release of the new specifications is almost two years now but the following is the highlights of the changes. There is 16 noted changes in the USGS NGP LBS V1.2 specifications as noted on page 3 of the Introduction. The main changes worth noting are that the accuracy assessment is using Non-vegetated vertical accuracy (NVA) and vegetated vertical accuracy (VVA) points as defined in the ASPRS Positional Accuracy Standards for Digital Geospatial Data and it is no longer required to use Fundamental Vertical Accuracy (FVA) Supplemental Vertical accuracy (SVA) and Consolidated Vertical Accuracy (CVA) analysis. The computation is the same for deriving the accuracy but the report is different because there is no break out in land cover classes, other than non-vegetated areas and vegetated area. This makes a lot of sense and helps keep the cost of survey down and more reasonable. Another notable change is the adoption of Quality level (QL) segmentation.

Basically, The QL gives you a general required density by points per meter (PPM) and accuracy of the QL to follow for a project. This is in terms of the actual accuracy of the data and refined in the USGS specification for the relative accuracy of the data as they relate to the QL. Typically, The USGS wants to have a QL2 or better for all their 3DEP collections. This is 2-4 PPM and an accuracy of 10cm RMSE and aligns with the ASPRS 10-cm vertical class. The QL2 for USGS is changed from 9.25 to 10cm. Why this was done is a question that might be asked but it makes sense. The LiDAR point cloud data is required to be delivered in LAS 1.4 Format. This is a major change as the 1.4 format can’t easily be converted from the LAS 1.2 format like LAS 1.2 could be from the other previous formats. The previous version of the USGS specification mentioned that overlap and withheld bit flags be required but it was "requested" for lack of a better term. They wanted it but the data would not be rejected if it wasn’t there. Now it is required. Previous the USGS was also flexible and sensitive to the way suppliers classified data pertaining to classes used but decided to make sure that bridges went into class 17 and high noise in Class 18. In terms of acquisition the requirements for flight line overlap and scan angle limits where removed but most suppliers will still be very cautious on this two variables and mostly on the removal of the 32 to 40 degree scan angle limitation but the relative error budget and data accuracy will dictate this and in most cases the suppliers will be forced to maintain these scan angles if they wish to get their data approved. The major reason for this is that the prism mirror systems can collect just as good data and the oscillating mirror systems at 60 degrees.

There has been some discussion on how to deal with the overlap bit-flag and how this should be done because we don’t live in a perfect world and there is very rarely a perfect project with a prefect rectangle that has a limited length that prevents the potential of IMU drift. Typically, the maximum flight line distance would be 80 Miles but usually professionals keep the distance between 40 and 60 miles. That being said, how should the overlap bit-flag be done for areas when two flight lines end on each other or a flight line end crosses over another at a perpendicular angle or at varying angles? What if 3 flight lines intersect each other? How should this be handled? When flight lines are parallel to each other this is easy and straight forward but when the cases previously presented is it necessary and acceptable to not assign an overlap bit-flag for these points used? This is more relevant now that the point density computation requirement has be changed.

There is also been a lot of conversation about the well-known-text (WKT) requirement as it relates to the current specification and how to handle that. There is WKT OCG compliant and some other versions of this such as the WKT ISO complaint. The two main types are WKT and WKT 2 and it would appear that the requirement is a Hybrid of the WKT and WKT2. This is basically, as understood at the time of the article basically the WKT OGC standard with the axis component of the WKT2. USGS recently released their requirement with much discussion and work on it so it is recommended that this document be reviewed in its entirety to have a full understanding of that is required. Continued discussion would be recommended as well.

In the past when the USGS specification has changed, it has been difficult and there has been several months of time lapse for some software provides to comply with the new specification. Software changes are required but takes some time to go into thousands lines of code to make sure the software is complying with the new specifications because ultimately the LiDAR providers are going to be using this software to provide data as required to the end-users. The end-users and the Quality Assurance professionals are very educated in the specification processes to make sure everything complies as required. It was relatively easy for most LiDAR providers to adapt to changes from the first LAS version up to LAS 1.2. There are a lot of smart professionals out there that have made this process almost transparent. This was not the case when going from LAS1.2 to LAS1.4 but it is possible to do. The XYZ information can be transferred with little change. The intensity information would need to be converted from 8-bit to 16 bit information, and 12 bit information that has been represented as 8-bit would have to go to 16-bit, which has been done for a while in the imaging world. The scanner channels would have to be converted to bit values and the classes would need to be changed to what is required in the LAS1.4 format. The hardest variable is the scan rank would have to be converted to the scan angle with the precision as required by the new format. This is possible but much programing was required to do this. Lastly the coordinate reference systems would need to be refined using WKT and as resolved by USGS recently. So the short answer is if you have LAS1.2 format data that meets the previous USGS specifications it can be converted to LAS 1.4 data if necessary without a total reprocess from scratch.

The changing of specifications is a necessary part of our profession and especially when it pertains to providing individual end-users better information to get the most out of their data. It is apparent that the individuals involved in the USGS specification had done an excellent job with this given that most everyone that procures data uses all or some part of their specification. Obviously, some applications are not conducive to this specification. As a necessary part of our profession most all of the providers of data have adapted and continue to adapt as required and it is expected that with the innovation and advances of LiDAR technology this will continue.

James Wilder Young (Jamie) CP, CMS-L is currently a Senior Geomatics Technologist for Merrick & Co. located in Greenwood Village, Colorado. His experience includes all aspects of LiDAR including sensor development, applications development, data acquisition, data processing and project management.

A 7.834Mb PDF of this article as it appeared in the magazine complete with images is available by clicking HERE