Datum conversions using NTv2 grids

Martin Isenburg

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Nov 24, 2020, 7:54:28 AM11/24/20

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Hello,

A new feature coming up in LAStools is the ability to do horizontal datum transforms using NTv2 grids. Anyone in need of doing this regularly? I would like to test a few different grids from around the world. If this interests you, please send me a link to three things:

(1) your *.gsb grid file
(2) a (small) LAZ file in one datum
(3) the same (small) LAZ file in the other datum

Regards,

Martin

Michael Stimson

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Nov 24, 2020, 7:15:22 PM11/24/20

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https://drive.google.com/drive/folders/14qgEEb7hmi6VdB6bc66_4UwD5C3M6i-J?usp=sharing

One small LAS file in both GDA94 and GDA2020 with NTv2 GSB used, sourced from ICSM https://www.icsm.gov.au/datum/gda-transformation-products-and-tools/transformation-grids the horizontal difference in this region is about 1.7 metres.

Michael Stimson

Senior GIS Analyst
RPS | Australia Asia Pacific
D +61 7 3539 9694
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Subject: [LAStools] Datum conversions using NTv2 grids

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Sonya Webb

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Nov 24, 2020, 8:08:29 PM11/24/20

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Hi,

Is there a way with the tools to identify what projection they are in?

Thanks,

Sonya Webb, P.E., CFM

Civil Engineer

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1672 Donlon St. Ventura, CA 93003

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Michael Stimson

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Nov 25, 2020, 6:01:47 AM11/25/20

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For LAS 1.2 and 1.3 embedding of coordinate system is possible but not mandatory, ASPRS LAS 1.4 however demands that coordinate reference system is defined in the header and must be WKT for PDRF 6-10.

If the CRS information has been defined LASInfo will reveal it; GeoTIFF tags can be a little difficult to decipher even if they are defined, WKT is much easier to read and will contain the projection name and parameters.

Michael Stimson

Senior GIS Analyst
RPS | Australia Asia Pacific
D +61 7 3539 9694
E michael...@rpsgroup.com.au

From: 'Sonya Webb' via LAStools - efficient tools for LiDAR processing <last...@googlegroups.com>
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Subject: RE: [LAStools] Datum conversions using NTv2 grids

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Martin Isenburg

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Nov 25, 2020, 6:51:44 AM11/25/20

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Nice trick, Michael, in trying to confuse me.

Naming the first file as if it was in GDA94 MGA Zone56 (epsg code 28356), but actually providing a file that is GDA94_MGA Zone55 (epsg code 28355). It did throw me off for a second and I questioned my code and re-run everything ... but then one of my laser chickens said "Cock-a-doodle-doo" and it woke me up to seeing the obvious input error.

lasinfo -i GDA94_MGA_Zone56_EPSG28356\GDA94_LiDAR2018-C4-ELL_5167035_56.LAS
reporting all LAS header entries:
file signature: 'LASF'
file source ID: 0
global_encoding: 17
project ID GUID data 1-4: 00000000-0000-0000-0000-000000000000
version major.minor: 1.3
system identifier: ''
generating software: 'TerraScan'
file creation day/year: 199/2019
header size: 235
offset to point data: 943
number var. length records: 1
point data format: 1
point data record length: 28
number of point records: 93260
number of points by return: 93260 0 0 0 0
scale factor x y z: 0.01 0.01 0.01
offset x y z: 0 0 0
min x y z: 516000.00 7035000.00 42.94
max x y z: 516078.72 7035411.55 43.85
start of waveform data packet record: 0
variable length header record 1 of 1:
reserved 0
user ID 'LASF_Projection'
record ID 2112
length after header 654
description 'OGC WKT'
WKT OGC COORDINATE SYSTEM:
PROJCS["GDA94 / MGA zone 55",GEOGCS["GDA94",DATUM["Geocentric_Datum_of_Australia_1994",SPHEROID["GRS 1980",6378137,298.257222101,AUTHORITY["EPSG","7019"]],TOWGS84[0,0,0,0,0,0,0],AUTHORITY["EPSG","6283"]],PRIMEM["Greenwich",0,AUTHORITY["EPSG","8901"]],UNIT["degree",0.01745329251994328,AUTHORITY["EPSG","9122"]],AUTHORITY["EPSG","4283"]],UNIT["metre",1,AUTHORITY["EPSG","9001"]],PROJECTION["Transverse_Mercator"],PARAMETER["latitude_of_origin",0],PARAMETER["central_meridian",147],PARAMETER["scale_factor",0.9996],PARAMETER["false_easting",500000],PARAMETER["false_northing",10000000],AUTHORITY["E
PSG","28355"],AXIS["Easting",EAST],AXIS["Northing",NORTH]]
reporting minimum and maximum for all LAS point record entries ...
X 51600000 51607872
Y 703500000 703541155
Z 4294 4385
intensity 89 10697
return_number 1 1
number_of_returns 1 1
edge_of_flight_line 0 1
scan_direction_flag 1 1
classification 7 9
scan_angle_rank -23 3
user_data 0 0
point_source_ID 201 265
gps_time 530019.921847 530022.068024
number of first returns: 93260
number of intermediate returns: 0
number of last returns: 93260
number of single returns: 93260
overview over number of returns of given pulse: 93260 0 0 0 0 0 0
histogram of classification of points:
61 noise (7)
93199 water (9)
+-> flagged as withheld: 54
+---> 54 of those are noise (7)

====================

lasinfo -i GDA2020_Zone56_EPSG7856\GDA2020_LiDAR2018-C4-ELL_5167035_56.LAS
reporting all LAS header entries:
file signature: 'LASF'
file source ID: 0
global_encoding: 17
project ID GUID data 1-4: 00000000-0000-0000-0000-000000000000
version major.minor: 1.3
system identifier: ''
generating software: 'TerraScan'
file creation day/year: 199/2019
header size: 235
offset to point data: 699
number var. length records: 1
point data format: 1
point data record length: 28
number of point records: 93260
number of points by return: 93260 0 0 0 0
scale factor x y z: 0.01 0.01 0.01
offset x y z: 0 0 0
min x y z: 516000.60 7035001.38 42.94
max x y z: 516079.32 7035412.93 43.85
start of waveform data packet record: 0
variable length header record 1 of 1:
reserved 0
user ID 'LASF_Projection'
record ID 2112
length after header 410
description 'OGC WKT'
WKT OGC COORDINATE SYSTEM:
PROJCS["GDA2020_MGA_zone_56",GEOGCS["GDA2020",DATUM["GDA2020",SPHEROID["GRS_1980",6378137,298.257222101]],PRIMEM["Greenwich",0],UNIT["Degree",0.017453292519943295]],PROJECTION["Transverse_Mercator
"],PARAMETER["latitude_of_origin",0],PARAMETER["central_meridian",153],PARAMETER["scale_factor",0.9996],PARAMETER["false_easting",500000],PARAMETER["false_northing",10000000],UNIT["Meter",1],AUTHORITY
["EPSG",7856]]
reporting minimum and maximum for all LAS point record entries ...
X 51600060 51607932
Y 703500138 703541293
Z 4294 4385
intensity 89 10697
return_number 1 1
number_of_returns 1 1
edge_of_flight_line 0 1
scan_direction_flag 1 1
classification 7 9
scan_angle_rank -23 3
user_data 0 0
point_source_ID 201 265
gps_time 530019.921847 530022.068024
number of first returns: 93260
number of intermediate returns: 0
number of last returns: 93260
number of single returns: 93260
overview over number of returns of given pulse: 93260 0 0 0 0 0 0
histogram of classification of points:
61 noise (7)
93199 water (9)
+-> flagged as withheld: 54
+---> 54 of those are noise (7)

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Michael Stimson

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Nov 26, 2020, 5:10:09 AM11/26/20

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Doh!

“A rose by any other name would smell as sweet”, you’re right it should be zone 56.

Most of the data I work on is in zone 55, a zone 56 project is a rarity, the incorrect definition was reflexive - part of a batch file or python script that I hadn’t changed the parameter on. Sorry, I wasn’t trying to deliberately mislead you.

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Martin Isenburg

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Nov 29, 2020, 7:38:16 AM11/29/20

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Hello,

las2las -i GDA94.LAS -epsg 28355 -target_longlat -odix _longlat -olaz

lasdatum -i GDA94_longlat.laz -grid GDA94_GDA2020_conformal.gsb -odix _gda2020 -olaz

las2las -i GDA94_longlat_gda2020.laz -target_epsg 7855 -ocut 16 -odix _7855 -olaz

lasview -faf -i GDA94.LAS -i GDA2020.LAS -i GDA94_7855.laz

Once I work entirely in MGA Zone 55 the sequence of commands shown above does the desired transform correctly and afterwards my computed GDA94_7855.laz perfectly lines up with your provided GDA2020.LAS

Regards,

Martin

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Seweryn Cieslik

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Jan 7, 2021, 8:03:58 AM1/7/21

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Good Morning and a Happy New Year!

@Martin, is there an ETA for the NTv2 shift release?

Regards

Sev

Martin Isenburg

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Jan 7, 2021, 8:20:19 AM1/7/21

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Hello Seweryn,

yes. The first version is ready and already in beta testing. You want to try it out on your data? Please send me a private email at lasd...@rapidlasso.com and I will provide you with access to the current version.

Regards.

Martin @rapidlasso

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