OxTS Georeferencer Full Guide

This section is a brief guide to setting up your equipment to perform a LiDAR survey. For hardware integrations please see the articles linked above. OxTS offers multiple cable types with LiDAR adapter interfaces for use with Velodyne LiDAR, these can be seen in the manual.

You will need to place the INS and LiDAR device on or in your vehicle with the LiDAR having an appropriate view. It is very important at this stage that you make sure both devices are secure and do not move during the survey. It is okay if the LiDAR is viewing some of the vehicle, this can be removed in processing later. The next step is to measure the positional and angular offset. 

Relative measurements: When you set up your configuration you will need to measure the relative distance between the INS measurement point and the LiDAR optical centre. Make sure to do this accurately and that you measure from the measurement point of each device not just the centre of their housings. These measurements include the XYZ linear displacement and also their relative roll, pitch and yaw. This is in addition to the INS-vehicle configuration measurements that NAVconfig requires. These measurements won't be needed during set up but note them down for Georeferencer later. If you want centimetre accuracy you will have to ensure the XYZ displacement is accurate to better than a centimetre and your angles to points of a degree unless you are boresighting which will take of this for you.

Pay close attention to the instructions later on for inputting the correct angle and displacement values.

It is particularly convenient to use a fixed mounting if you want to take many surveys and to be more accurate in your measurements. If you have a standard, repeatable configuration your life will be much easier as you only need to take configuration measurements once and the setup only needs to be boresighted once. CAD files of the LiDAR will be available from the manufacturer and of the INS from OxTS. However, with our boresighting calibration you can work much more flexibly if required.

Figure 1: A mount that OxTS uses internally. It has a fixed relative position and orientation and holds both devices securely to the vehicle. On the right is a view of the setup that can be seen in Georeferencer (old version of Georeferencer has been used, the new version displays a blue cube).

There are many different ways of setting up your cable connections depending on your preferences. Below, on the left, is a diagram of the setup for taking data using a Velodyne LiDAR. On the right is a selection of choices for viewing data. These are discussed more thoroughly in the next section. You can choose setup 1 or 2 to view both streams of data in real time or setup 3 to see only one stream of data or to setup the INS. Your computer may require USB-ethernet adapters. Bear in mind that if you are using PTP for time-syncing the switch also should be PTP-compatible.

Figure 2: Diagram of a device setup. On the left, the setup of cables to run the devices and log data. On the right, setup for viewing data in real time.

IP configuration: If you are viewing data in real time you will likely need to configure your laptop's IP addresses for the incoming INS and LiDAR data. If you have streams of data coming in from multiple devices you will have to make sure your PC is configured to be on the correct IP range as each one. This can easily be done by right-clicking on the internet icon in the lower right on the taskbar for a Windows 10 PC and choosing 'Open Network and Internet Settings' then 'Change adapter options'. Right click on the ethernet connection bringing data in and choose 'Properties'. Click on 'Internet Protocol Version 4 (TCP/IPv4)' and then click 'Properties'. Choose 'Use the following IP address' and input an IP address on the correct range of one of the devices. For example for an xNAV with IP address 192.168.0.100 you can choose 192.168.0.52. Then click on 'Advanced' and add further IP addresses on the same ranges as other devices for example for a LiDAR with IP address 192.0.0.201 you can use 192.0.0.52. Having done this your PC will be able to handle viewing the INS in NAVconfig/NAVdisplay and the LiDAR in whichever software/web interface it uses. 

NAVconfig: You will have to put in all the necessary measurements into NAVconfig > Hardware Setup. In NAVconfig > Hardware Setup > LiDAR Scanner, select your scanner type and, if using onboard LiDAR logging, tick the boxes for logging data and logging telemetry. For onboard logging, you must first have the feature code for this enabled and second you must ensure that the data rate from the LiDAR you are using does not saturate the CPU; typically this applies for LiDAR with over 16 lasers. Version 3 OxTS INS devices can store up to 32GB of raw data (INS + LiDAR). Files logged onboard are saved as LCOM format. You then have a choice of sending your NMEA data over ethernet or over serial (see Figure 3). The LiDAR cable connection from OxTS for Velodyne LiDAR that supplies the LiDAR with power from the INS also sends the PPS and serial data. NMEA data can be sent over an ethernet connection using a connector, a switch or a network bridge. The default ports should not need to be altered. If you are sending data over ethernet you will need to put the IP address of the LiDAR device in, alternatively you can use the broadcast IP address 255.255.255.255 but this may unnecessarily saturate your network. Alternatively PPS and NMEA can be configured in the Interfaces tab of NAVconfig.

Figure 3: Selecting how NMEA data is transferred from the xNAV to the VLP (right highlighted box). Selected scanner type, inputting IP address and data logging settings (left box).

In NAVconfig > Environment you will need to select ‘Enable local coordinates’ and choose your origin (see Figure 4). This can be done in post-processing in NAVsolve > Process > Local coordinates or by creating an LRF file manually. Doing it before your survey will simplify the processing later.

Figure 4: Enabling local coordinates in NAVconfig, you can then choose your origin. 

If you are not using NAVsuite 2.8 and you choose ‘Send NMEA over serial 1’ as an option you will need to then go to NAVconfig > Interfaces > Serial 1 Output and ensure that GPGGA and GPHDT are switched off. 

PPS and NMEA: Now is also a good time to ensure that the hardware is working as expected. Particularly you should check that the LiDAR and INS are powered on as expected and data is sesnsible. Another good sanity check often is to probe the LiDAR to check if it is receiving PPS and NMEA messages. This can often be done in a web interface by typing the LiDAR IP address into a web browser or in the case of the Ouster using TCP commands (get_time_info). For example, if using a Velodyne product, you can use the Velodyne web interface to make some checks. Putting the IP address of the VLP into a web browser will bring up the web interface (see Figure 5), you can check there that the PPS is locked and that there is a real time GPS position (the LiDAR is receiving NMEA data). Additionally, you can open VeloView to see the LiDAR data in real time.

Figure 5: The Velodyne web user interface is accessed by typing the IP address into a web browser tab. Check that PPS is 'Locked' and the GPS Position is updating in real time (and so the LiDAR is receiving NMEA).

Data can always be viewed coming in with software such as WireShark, this is another good sanity check and troubleshooter to see that the correct data from the correct IP addresses is being received.

It is encouraged that you check that the correct data is being logged and to have a trial before starting your survey. It is also recommended that you have folders set up for putting data into as it comes in.

Initialisation: It is important that your INS is initialised while it is taking data, this is because initialisation is used as part of the synchronisation check between the LiDAR and NCOM data, NMEA messages also have a validity flag that is disabled for uninitialised data. Initialisation is when your INS device locks onto its location and heading. You are able to view in NAVdisplay if your system is initialised or is ready for initialisation. You do not have to begin your survey initialised but at least one third of the time your data file is recording you should be initialised. In NAVconfig > Environment you can set your initialisation settings to use static (requires dual antenna) or dynamic initialisation. For UAV use you will likely need static whereas for mobile use it is recommended that you select dynamic and ensure that you can drive in a straight line to initialise the device.

Figure 6: In the bottom right of the default template the 'NCOM Navigation Status' indicates if the system has been initialised. 

Warm up: It is most important that a suitable warmup is performed to get the device into spec. This takes about 10 minutes following initialisation and consists of doing a variety of manoeuvers to get the device's Kalman filter warmed up. To get the best output from your unit you can read here. Ideally you would want a long, exhaustive warmup and to make sure your setup is known as tightly as possible for the very best possible survey data. You can follow NAVdisplay to monitor the accuracies the INS is reporting.

LCOM logging: LCOM is essentially PCAP data recorded directly onto the INS, this is possible for 16 laser LiDARs or less due to the data rate. While taking your data, you may wish to break up your survey into multiple runs. This can be done easily using NAVdisplay. If you are viewing your INS data in real time then you can click in the command box at the bottom of NAVdisplay and type "!log log on" and "!log log off" and then click send to stop and start data logging if using onboard logging. The RD file will continue to log as it runs in a separate process of the firmware but the LCOM will stop, you can view this happening by using an FTP connection and seeing that the file size does not grow after refreshing. You can use the same RD file for multiple surveys later. LCOM files are identical to PCAP except that ethernet headers are not saved. 

PCAP logging: WireShark is particularly useful for recording data. If using WireShark, select the incoming stream of data (e.g. Ethernet 2) and you can view the LiDAR and INS data coming in. Right click the IP address of the LiDAR and then choose filter on selected to see only the LiDAR data. Stop and start the data stream as you wish to survey and save these as PCAP files (WireShark's default file type is PCAPNG). If using Ouster LiDAR, packets that come into WireShark need to be reconstructed. This can be done but it is recommended that OusterStudio is used for recording Ouster data. Similarly, other LiDAR manufacturers provide software that can record PCAP in suitable formats. 

A single RD file is usually sufficient for the whole surveying day with multiple surveying files taken at suitable times due to relative file sizes.

If you open OxTS Georeferencer you will see that you require 5 files in the Files tab. These are an NCOM file, a LiDAR and an LIP, LIR and VAT file (see Figure 7). All of these files are simple to obtain after completing your survey, they can also be available during your survey. You can simply drag your files from File Explorer into Georeferencer. Before processing you will then have to select from the drop-down box which LiDAR it was you used.

Figure 7: The Files tab in Georeferencer showing your 5 files and the NCOM journey on Bing maps.

NCOM

You will need information about your vehicle trajectory in order to georeference. This will be in an NCOM file. To obtain your NCOM file you will need to retrieve your raw data file from the INS and then process it into an NCOM. You can FTP to the INS via an ethernet connection and download any files you need, this can be done with File Explorer or Filezilla and you can then download the files to your computer. In File Explorer type ftp://192.168.1.xxx where the x's are the IP address of the INS (see Figure 8). The time that the data log started is recorded in the name of the file. When you have the RD file you will need to process it, this is done using our NAVsolve software and you can find a guide for processing your RD file here.

Ensure when processing the RD into an NCOM that local coordinates are enabled. This can be done in NAVconfig or in NAVsolve in Process > Local Coordinates. Check in NAVgraph that the data appears as it should, you can check the pitch and position accuracies to see if the device was in spec. 

Figure 8: Using an FTP connection (ftp://192.168.1.13) to the INS to retrieve the two most recent files in its data. You can use an FTP connection in real time to check that the INS is logging the RD and LCOM file by refreshing the File Explorer window and viewing the file size. The highlighted RD file was created on the 12th February 2020 (20/02/12) at 15:24 as seen in its filename.

LiDAR

The second file you need is a LiDAR file, this can be LCOM or PCAP. An LCOM file can be found on the INS via an FTP connection if you set up your devices for this. If you didn't setup your devices to record LCOM then you will have recorded a PCAP during the survey run via some software like Wireshark or Veloview. You will need to take this file from where you saved it and drag it into OxTS Georeferencer. 

The first check that OxTS Georeferencer makes is that the LiDAR data is PPS synchronized and has NMEA data, this will depend on if you have set up the device connections correctly. If the data passes this check you should see a tick. If the file fails synchronization, consider your setup again and check any connections and network configurations you have, you should be able to do this in real time (see above sections). Remember that the INS must be initialised for the LiDAR log to have the required synchronisation. To be able to process your pointcloud the NCOM and LiDAR data must align in time as well. Opening an NCOM in OxTS Georeferencer will show your journey route overlaid on Bing maps to help you check which survey the NCOM corresponds to. A pointcloud will be created for the time that the LiDAR and INS data overlap.

VAT

The VAT file is the angular configuration of the INS with respect to the vehicle frame. You must put in some preliminary measurements for this when setting up in NAVconfig but the system will intelligently improve the angles throughout the journey. You therefore do not have to make a VAT file yourself. After processing your raw data file the VAT file will be available in the "Process_..." folder or the "ConfigurationFromRawData" folder created by NAVsolve. The VAT file is a simple text file with a .vat extension.

LIP and LIR

The LIP file is the positional configuration of the LiDAR with respect to the INS frame. This is the x, y and z displacement from the INS measurement point to the LiDAR device, each of these measurements is on their own respective line in a simple text file with an .lip extension. You can create an LIP file by clicking on the LIP plus icon in Georeferencer and then input the numbers in the Hardware Configuration tab. You should have measured these while setting up and they should be as accurate as possible. Boresighting will calibrate these values for you.

For users used to using commas instead of points for a decimal point separator you must use a point. The next version of Georeferencer will accommodate for comma separators. 

Follow carefully the instructions in the next section.

LIR

The LIR file is the angular configuration of the LiDAR with respect to the INS frame. This is the relative yaw, pitch and roll displacement between the INS measurement axes and the LiDAR device axes (x, y, z), each of these measurements is on their own respective line in a simple text file with an .lir extension. You can create an LIR file by clicking on the LIR plus icon in Georeferencer just like the LIP.

Measure the angles as best you can between the orthogonal axes of the LiDAR and the INS. The INS axes are printed on the device and shown in Georeferencer. The axes for the LiDAR device will be shown in the user manual and they are typically different for each LiDAR. Ideally take photographs of your setup to troubleshoot this process especially if you want support from OxTS. 

The best way to do this is to make the angles between INS and LiDAR multiples of 90 or close to it. You can then make sure that the axes in the hardware tab on the LiDAR are where they should be. You will likely have to work out the correct combinations of rotations to get the correct setup, do this by performing each rotation one after other yaw, pitch and roll. 

Example of determining the LIR for a setup.

1. Check the manual for the LiDAR unit axes and visualise how these relate to the INS axes. For example, the y axis of the VLP16 points the opposite direction of the cable and the z axis points up. 
2. Work out the correct combination of rotation to rotate the axes of the cube into the correct LiDAR axes. To begin with a 0, 0, 0 LIR corresponds to having the same axes as the INS but we need to rotate the axes to reproduce the image on the right above.
3. This can be done by applying first a 180 yaw rotation followed a 90 pitch rotation and a 90 roll rotation so an LIR of 180, 90, 90 is used in Geoeferencer.

This same process is used for all different LiDAR.

• Hesai LiDAR:
  
• Ouster LiDAR:
  
• Velodyne LiDAR:
  

You can make estimations for the correct axes beforehand and then edit them in the Hardware tab of Georeferencer where you can view the configuration. In addition, if you have done a boresighting run, make sure you use the boresighted LIR values. A boresighted LIR file will have a flag value of '1' on a fourth line to signify it is optimised.

Example LIR, LIP and VAT files are available at the bottom of the page.

A future version of Georeferencer will have a CAD model for different LiDAR coordinate systems.

Figure 9: In the Files tab you can create the LIP and LIR files by clicking the plus icon. The files are then edited and viewed using the Hardware Configuration tab.

Figure 10: The hardware tab allows you to view your configuration and make alterations to the LIP and LIR files. Above compares the view from previous versions of Georeferencer to the new version (bottom) for the same setup.

JSON

If using an Ouster LiDAR then you will need to input a JSON file that you've taken from the unit. This can be done easily using OusterStudio and choosing the second icon down on the left. Save the JSON file in the same folder as the PCAP with the same name. Alternatively, use the TCP command get_beam_intrinsics and put the the output into a JSON file format.

Boresighting is OxTS’s in-built calibration method. When you make your angular offset measurements they can be quite difficult to make accurately. A small angular error can make a large difference to a Pointcloud and significantly lower the quality of your survey. We recommend you boresight your configuration before carrying out a survey; but once a setup is boresighted it will not need to be boresighted again. Boresighting will make your LIR and LIP file much more accurate and prevent blurred images or ‘double vision’ that may occur. A guide for boresighting can be viewed here.

Boresighting Reflectivity: The reflectivity threshold ought to be set to a value that will include only the boresighting targets and no other points. For most LiDAR this is calibrated to be around 100. This can safely be increased to say 150 should you see too many non-target points. To see what values of reflectivity points have, georeference your Pointcloud and heck the 'Intensity' field. Most ordinary points will not have intensities over 40.

Boresighting Options where you can select calibration type.

In the Boresight Options within the Processing Options section of Georeferencer you can choose LIR optimization, LIP optimization or both. LIR calibration will calibrate the angles in your setup and is very robust. LIP calibration is in beta mode and will be improved in following versions of Georeferencer. LIP calibration works very well in some cases and in some cases it does not work depending on the quality of the georeferenced data.

Figure 11: Example comparison of an unboresighted (left) and boresighted (right) LIR and LIP (orientation) file. The calibration is able to refine the orientation much finer than it is possible to do by sight.

Figure 12: Example of a Pointcloud before (left) and after (right) boresighting.

A useful feature for different applications is the ability to choose the distance range of points that OxTS Georeferencer will turn into a Pointcloud. This is done by changing the minimum and maximum parameters under 'Range Between which points will be written (m):' section in Files > Processing Options. If you want to only see a road for example and not surroundings far away then you can restrict the range to 0-10m. If you wanted to only see over a certain range, in some geographical application for example, you can set the range to 10-100m. If the LiDAR had some part of the vehicle visible in its field of view then put a minimum distance of two meters so the vehicle is not seen throughout the Pointcloud.

An article on using advanced commands in Georeferencer can be viewed here. To quickly view advanced commands in Georeferencer you can upload some files and then type in an invalid advanced command and click 'Run Georeferencing'. 

Reflectivity: Other useful options include changing the reflectivity threshold and the number of iterations that the boresight procedure goes through. If boresighting does not appear to work or it only works in one or two axes then increasing the number of iterations the software makes on the data can often fix the problem. The reflectivity threshold is perfect for troubleshooting if you do a boresight run and want to know why something isn't correct. Increasing this to 100 for a Velodyne for example will give only the targets in a Pointcloud only a few hundred kilobytes. 

Many of the advanced settings are achievable in Pointcloud post-processing software but due to the large file size it is often more useful to do it during the creation of the Pointcloud. 

PTP Time Offset: A time offset might have to be applied to the LiDAR time packet data to work with the navigation data when using PTP. This is added by an advanced command. Use the 'lidar_time_offset=x' advanced command to add x seconds to the time packets of the LiDAR data. For use with Hesai data for example use 'lidar_time_offset=315964804000000000'.

Error threshold: Most importantly, the 'Error threshold' option will allow you to only process data into the Pointcloud that is above a certain threshold of uncertainty. If you wish for all points in your Pointcloud to be above say 8cm accurate then entering 8 into the Error threshold will give a good estimation of this. This error data is shown in the UserData field that is viewable in Pointcloud viewing software such as CloudCompare so you can see the uncertainties of each point in a section of a survey based on the navigation data.

Figure 13: A survey along a road. In red, data can be seen where the surveyor lost RTK accuracy, this section can be removed using the Error threshold option or in Pointcloud viewing software to give only the most accurate data.

The error estimation uses a sophisticated formula produced by the American Society for Photogrammetry and Remote Sensing (ASPRS) that combines LiDAR uncertainty and NCOM uncertainty to estimate the absolute uncertainty of point positions within a Pointcloud. NCOM uncertainties are themselves estimates that the INS outputs. These uncertainties can be viewed in real time in NAVdisplay and are based on simulations. Contributions from boresight misalignment are not considered and the values are modelled on the accuracy of a VLP16 LiDAR, other LiDAR units might be more or less accurate in their range but they will likely be similar and the main contributions from the IMU.

The highest uncertainty estimate, which will appear as dark red as standard in software such as CloudCompare, is set to 50cm which is representative of going into SPS mode. 

NCOM outputs accuracy diagnostics as calculated uncertainties in all conditions in real time and these are used to estimate the uncertainty in the points' positions. These outputs include estimations of the uncertainty in heading, pitch and roll, North and East position and altitude and all of these are combined in the formula.

The following tables give the translation between the UserData field value between 0-255 and the cm position uncertainty of the points given by the formula. Also included are the ranges at which these uncertainties are achieved for a unit that is working at specification in good GNSS conditions (open sky and no obstructions):

xNAV

Survey+

With your 5 compatible files you will be able to click Run Georeferencing. This will create a folder in the directory that the NCOM data is in or where you specify in the Files tab; the folder will contain your Pointcloud, the LIP, LIR and VAT used and 2 processing logs.

Georeferencer can create Pointclouds in LAS, LAZ (compressed) or PCD formats. This is chosen in the Files > Processing Options tab. The default is an LAZ file.

You will now be able to view your cloud in your choice of Pointcloud viewing software (eg CloudCompare, QT reader or others).

If you would like example sets of data please get in touch or download straight from the website. We also appreciate you sharing your data with us.

Figure 14: Example of cloud being viewed in CloudCompare after being processed in OxTS Georeferencer.

This section will cover some problems that one can encounter while using Georeferencer and some potential solutions.

• Other objects during boresighting: If your LiDAR unit is not calibrated to have retroreflective objects at a registered reflectivity of 100 (out of 255) then the default boresight calibration might not work. If this is the case then you can select 'processing options' before calibrating and change the 'reflectivity threshold' on under 'boresight options' to a higher number e.g. 150. 
  
  Figure 15: Example clusterplot with the default reflectivity threshold of 100 using a VLP32. Instead of just the targets many objects show up.
• Target smearing: If your setup or navigation data is poorly configured then the targets may be too smeared out to correctly boresight. When choosing the locations of the targets on the clusterplot page you must ensure that the blue circle encompasses all of the points that correspond to a target and also that no other retroreflective points are encompassed in the circle. If you can't make this happen you may have to manually measure your setup better until it is good enough to boresight. 
• Pointcloud 'lite': A common technique for troubleshooting boresighting is to produce a light pointcloud by setting reflectivity to 100 and running georeferencing.
• Time overlap: If the navigation (NCOM) and LiDAR (PCAP) files that you have chosen do not overlap in time Georeferencer will not be able to produce a pointcloud. You should doublecheck therefore that the files you are trying to use are the correct ones. If they are then you might have a more fundamental problem on the hardware level. You should receive an error message when you attempt to process that indicates your files were not taken at the same time. You might receive an error looking like "Reached end of LCOM stream at time: 1285502583250000000
  FAT:G.1.3: Failed before attempting georeferencing".