Fixing LAS timestamps

In some cases hardware manufacturers and/or software are not properly setting the GPS Timestamps in an LAS file according to the LAS specification. That specification dictates that the timestamp for LIDAR points be stored in either weekseconds or Adjusted GPS Standard time. This is very important as the tools within TerraScan and other software expects the time in one of these formats in order to properly align the data with other data types. For instance, as the sensor trajectory when checking or modifying system calibration, or cutting overlap. To assist users, TerraScan introduced a tool that could be used to “fix” the timestamps associated with LIDAR point data. Since this is a nonstandard tool and the problem with the timestamps really falls on the software which created the data file in the first place, it doesn’t appear in the standard routines or tools menus, buttons or macro steps. The tool first existed as a key-in “development fix time x”, where x represented the value in seconds to be added to the current timestamp value. Hence, if my data was off by a day and showing a Monday timestamp instead of Tuesday, I would key-in “development fix time 86400”. This would advance the existing timestamp by 86400 seconds.

As various data files have been encountered with this issue and the prevalence of them in the industry very high, and the resolution of this issue in some of the producing software very slow an expression, FnScanFixTime(x), where x is defined as above, was added such that the fixing of timestamps could be applied to dataset through a macro. Note that neither of these methods are the standard way of converting between the known timestamp formats of the LAS specification. For that one would want to use the Convert Time Stamps options when reading in or importing LAS data, or in the trajectory or image list dialogs.

For example, you have a dataset where the trajectory time is GPS week-seconds, but the laser data is UTC and not one of the standard LAS time formats. Hence, there is a need to add x*24*3600+16, where x is the number of days in the week and 16 is the leap second offset which is 16 seconds at the time of the acquisition. In this dataset x=5, so add 432016 seconds.

Fix Time Macro Step

Fix Time Macro Step


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