FAQs

General

Processing with USGS/Publicly Available DEMs

For systems equipped without LiDAR, USGS has a published site with publicly available DEMs for much of the continental United States. To get started:

Download DEM

  1. Visit USGS LidarExplorer

  2. Move to the DEMs tab

  3. Enable Show where DEMs exist?

  4. Navigate to the location of your survey and draw an AoI around the area

  5. Expand the DEMs within AoI dropdown in the right panel

  6. Select the highest resolution DEM available (typically 1 meter)

  7. In the new tab opened, scroll to the TIFF download link and download the DEM

To ensure compatibility with field sensor data and GRYFN Processing Tool, the vertical datum must be converted properly. GRYFN recommends using the NASA Ames Stereo Pipeline, with instructions below.

Converting DEM Vertical Datum

  1. For Windows users, install Windows Subsystem for Linux

  2. Open Windows terminal

  3. From the dropdown menu, select Ubuntu (or other Linux distro)

    1. First time users will need to create a username and password for Linux

  4. First time users should run the following commands to install NASA Ames Stereo Pipeline

    1. wget https://github.com/NeoGeographyToolkit/StereoPipeline/releases/download/3.5.0/StereoPipeline-3.5.0-2025-04-28-x86_64-Linux.tar.bz2

    2. tar -xvf StereoPipeline-3.5.0-2025-04-28-x86_64-Linux.tar.bz2

  1. cd StereoPipeline-3.4.0-2024-06-19-x86_64-Linux/

  1. LD_LIBRARY_PATH=$PWD/lib bin/dem_geoid --help

  1. LD_LIBRARY_PATH=$PWD/lib bin/dem_geoid /mnt/{path_to_dem}.tif --reverse-adjustment

    1. Reverse AdjustmentTransforms DEM from MSL to NAD83 Ellipsoid Height

  1. ls

    1. To locate new filename

  1. mv ./{newFilename} /mnt/{newFilePath}

Can I process RGB, VNIR, or SWIR data if I do not have LiDAR data/Point Cloud or DSM captured at the same time?

Yes, RGB and VNIR data can be processed without LiDAR data from the same flight in different ways, each with varying implications on orthorectification quality:

  1. For both RGB and VNIR, you can use a DSM from a different date (as close in date as possible to the flight you want to process). Edit or create a pipeline with an RGB or VNIR task. For your mosaic product, under Digital Surface Model, select Select Other and browse for a DSM from a different date of that site.

  2. Publicly available DSMs, from sources such as USGS, may also be used. See the FAQ "Processing with USGS DEMs" above.

  3. An additional method is to use an RGB generated sparse point cloud. To create a Sparse Point Cloud, edit or create a pipeline with an RGB task. For your mosaic product, under Digital Surface Model, select Sparse Point Cloud. VNIR and RGB DSM options will allow you to select the newly defined RGB sparse point cloud.

  4. Hyperspectral and RGB tasks have the option to use a fixed elevation value. If the survey area is relatively flat, a fixed value above ellipsoid can be set in the processing configuration in place of a DSM or point cloud. The value should match the nominal height of the scene in the same reference frame as the GNSS-INS export trajectory (WGS84 height above ellipsoid by default).

Can we use or do we need to use ground control points (GCP)?

While GCPs can be a helpful tool for confirming georeferencing accuracy of post-processed data products using GIS software, GRYFN Processing Tool does not currently support the input of GCPs. GRYFN remote sensing systems are integrated with a high-quality GNSS/IMU sensor which enables products to be processed with a georeferencing accuracy of 2-5cm, therefore eliminating the necessity of GCPs for well-located data.

I want to use my license for GRYFN Processing Tool on a different PC

As of v1.7.5, GRYFN Processing Tool licenses can be released in Settings\Product Licensing for re-licensing on a new machine.

Mingw Error

Windows error, requires restart of PC.

GNSS

POSPac Leap Second Error

Applanix has issued two separate service bulletins regarding ephemeris data download and a leap second error that may appear in POSPac versions < 9.4.

Ephemeris Service File Source Change

Introduction

POSPac requires service files, such as broadcast and precise ephemeris data, for accurate GNSS-Inertial post-processing, particularly for IN-Fusion SmartBase and IN-Fusion+ Single Base modes. Applanix leverages service files from various sources, including unique files from the Trimble service pool. The source for Trimble’s service files will change on July 1st, 2025.

Remedy

POSPac v9.4 and later versions have already adapted to the new database source. Therefore, no user action is required. Users of POSPac v9.x (excluding v9.4) must replace the “EphemDataServices.ini” file located in the installation directory (C:\Program Files\Applanix\POSPac MMS 9.x). This file can be downloaded from the Applanix Support Hub. This workaround may function for older versions (e.g., v8), but official support is limited to POSPac v9.

Leap Second Error on Import

Background

Starting in July 2025, the Trimble ephemeris files that are downloaded by POSPac may intermittently not include the correct leap second information. POSPac will then show the following error, as it is unable to determine leap second information on its own:

“The selected GNSS data file cannot be imported because it does not include accurate leap second information”

Solution

To resolve this issue, please download the LeapSecond.xml file and use it to replace the existing file in the following location on the processing computer: C:\ProgramData\Trimble\LeapSecondsConfig

APX SBET does not closely match raw data, "randomly" shoots off in one direction partway through flight

  1. Open Project Settings -> GNSS-Inertial Processor -> Initialization

  1. Change GNSS Track Heading option

    1. Use different Initial Altitude Source settings selected

  1. Re-do GNSS Inertial Processing step and analyze SBET

  1. Repeat with different GNSS Track Heading options until SBET closely resembles realtime trajectory

LiDAR

LiDAR Fatal Exception: EXCEPTION_ACCESS_VIOLATION

LiDAR may not have proper boot delay

  1. Open first PCAP in Wireshark

  2. Check for GPRMC codes in packets

    1. GPRMC will start showing up in packets before all info is available. Look for first GPRMC message with full info

  3. Select all packets before proper GPRMC message

    1. Edit -> Ignore Packet

  4. File -> Export Specified Packets -> Check Remove Ignored packets

What is the finest resolution DSM I can generate?

LiDAR range accuracy of Velodyne and Ouster 3D Rotating LiDAR sensors GRYFN integrates is ~2-3cm. Processing finer resolutions than 2-3cm may not result in greater surface detail, and may begin introducing excess noise to the surface model.

Should finer resolution surface models beyond the range accuracy of the sensor be desired, GRYFN recommends generating a 4cm surface model and doing interpolation to generate finer resolutions.

It should also be noted that as surface model resolution becomes finer, Median Filter Radius should be adjusted accordingly. GRYFN uses a 5x ratio of surface model resolution to filter radius value. Exceeding 10x ratio may result in system memory issues, and can cause GRYFN Processing Tool to crash.

Hyperspectral

PPS File is Empty

Should only happen for legacy Nano HS systems that have not received a hardware PPS upgrade. If so, PPS file can be spoofed by using the IMU_GPS file.

  1. Make a copy of IMU_GPS and open it in Notepad++

  2. Remove the final column (0.000000000000000)

    1. Click in front of the first 0 in the column

    2. Scroll to bottom of document

    3. Hold Ctrl+Alt+Shift and click the end of the final column

    4. Hit backspace or delete to remove the column

  3. Remove all lines in the file that are not on a perfect second

    1. Locate a line with timestamp of HH:MM:SS.000000

    2. Highlight just the .000000

    3. Ctrl+F

    4. Go to Mark tab -> select Bookmark line checkbox -> Mark All -> close the popup window

    5. Search -> Bookmark -> Remove Unmarked Lines

    6. If there are any lines that have a set of 0s like this not in the HH:MM:SS column, remove those lines manually

    7. Search -> Bookmark -> Clear All Bookmarks

  4. Copy the Timestamp column into Excel (column right before the YYYY/MM/DD column)

  5. Follow the above steps for an IMU_GPS file from a flight with a good PPS file as well

    1. Take the IMU_GPS and PPS file from a flight of that sensor that does have PPS data and determine the average difference of the timestamps from IMU_GPS vs PPS (IMU_GPS - 3200 worked for me on nHS-211)

    2. You will need to remove excess rows from the good flight's IMU_GPS before calculating the average difference

  6. Use the difference to alter your IMU_GPS timestamps from your flight missing PPS data and save that in the PPS file.

  7. Process away

Misalignment between adjacent flight lines

Typically this happens when PPK trajectory quality is poor. Try reprocessing GNSS data, or inspect the logs/reports from GNSS processing to assess quality.

If GNSS data is okay, and the issue is continuous, this could be indicitave of a calibration issue. Please reach out to support@gryfn.io for investigation and next steps.

Mosaic has non-orthogonal streaks

Alignment data was likely captured; altitude estimations and flight line detection did not properly remove this data. Check the ortho heatmap for streaks that are not flight lines. Check each raw data cube PNG, notice the length of the cube and if it is especially "wavy" relative to other cubes from inside the dataset.

RGB

More RGB images than events (aka missing events)

RGB images may have been taken before the APX was aligned, so there may be images from takeoff or alignment. Possibly images during final alignment and landing as well.

Each option below is an attempt to remove extra images.

  1. Count number of images taken on the ground before flight and see if that matches the number of missing events. If so, remove these images and attempt to re-process.

  2. If there are more missing events than just ground images, this process may need more manual involvement and can be time consuming.

    1. Open the GPRO\APX trajectory.json file in QGIS.

      1. Add in the event points and trajectory line data.

    2. Open the RGB images in a photo viewer.

    3. Run through the images manually in photo viewer and see if you can determine where images exist that do not have events.

    4. Remove these images that do not have events.

Extra events (vs # of images)

  1. Determine difference in number of events vs number of images.

    1. Open event1.dat in processed APX folder

    2. Remove the extra number of events from the end of this file

    3. Re-process

    4. If this doesn't work to fix the mosaic, move on to next option

  2. This process may need more manual involvement and can be time consuming.

    1. Open the GPRO events file in QGIS.

    2. Add in the event points and trajectory line data.

    3. Open the RGB images in a photo viewer.

    4. Run through the images manually in photo viewer and see if you can determine which images matches with each event, and remove any extra events from the event1.dat file.

Mosaic is rotated relative to actual spatial layout

Most likely cause is extra events. Extra events can cause a mosaic to be rotated because each RGB image could be matched up with an event with improper location and rotation information. Features will still be matched as normal, and could even be stitched properly, but the mosaic as a whole will rotate and/or shift if events are matched to the wrong images.

Inspect the number of images and number of events. If there are extra events, you may inspect where extra events were captured that do not have images and remove them from the event1.dat file from the GRAW.

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