LiDAR PDAL – Addressing Precision Loss When Importing LAS Files with PDAL

laslidarpdalpgpointcloud

I check pdal info and see that bbox has 15 digit numbers:

"boundary": { "type": "Polygon", "coordinates": [ [ [ 1528634.540273188613355, 403421.710437842586543, 234.984 ], [ 1528634.540273188613355, 407979.483437842573039, 234.984 ], [ 1528834.540273188613355, 407979.483437842573039, 315.504 ], [ 1528834.540273188613355, 403421.710437842586543, 315.504 ], [ 1528634.540273188613355, 403421.710437842586543, 234.984 ] ] ] }

Then I imported this las file into pgpointcloud with this pipeline.json:

{
        "pipeline": [
            {
                "type":"readers.las",
                "filename":"%s"
            },
            {
                "type":"filters.chipper",
                "capacity":400
            },
            {
                "type": "writers.pgpointcloud",
                "connection":"%s",
                "table":"%s",
                "compression":"dimensional",
                "srid":"0",
            }
        ]
        }

The scheme in pointcloud_formats:

<pc:dimension>
  <pc:position>13</pc:position>
  <pc:size>8</pc:size>
  <pc:description>X coordinate</pc:description>
  <pc:name>X</pc:name>
  <pc:interpretation>double</pc:interpretation>
  <pc:active>true</pc:active>
 </pc:dimension>
 <pc:dimension>
  <pc:position>14</pc:position>
  <pc:size>8</pc:size>
  <pc:description>Y coordinate</pc:description>
  <pc:name>Y</pc:name>
  <pc:interpretation>double</pc:interpretation>
  <pc:active>true</pc:active>
 </pc:dimension>

And finally coordinates in table

with pts as(
    select PC_Explode(pa) as pt
    from "Blocks"
    where id = 100000
)
select 
     PC_Get(pt, 'X') as "x"
    ,PC_Get(pt, 'Y') as "y"
from pts
order by PC_Get(pt, 'Z');
-------------------------
x                   y
1531507.46827319    404533.469437843
1531507.47027319    404533.577437843
...

So in imported table precision is 8 digit numbers.

After I added both, the table and the las file in QGIS and found that the same point very very close but not exactly the same.

So did I lose precision? And how to prevent this?

Best Answer

You can preserve the precision by setting the scale factors in the point cloud writer, try adding these just below "type": "writers.pgpointcloud" :

        "scale_x":"0.000000000000001",
        "scale_y":"0.000000000000001",

From the docs:

Function docs: https://pdal.io/stages/writers.pgpointcloud.html

Related Solutions

[GIS] Converting .las to .pcd file using PDAL

The PDAL PCD Writer can write .pcd files. For it to work, you must have linked the PCL libraries at compile time. The PDAL OSGeo4W build does not have PCL support. If you are unable to build PDAL with PCL support yourself, one possibility is to use PDAL's Docker containers to achieve your task -- those have PCL linked.

Then, simply run pdal translate:

Using your own PDAL build with PCL linked:

pdal translate input.las output.pcd

With Docker:

docker run -v /c/Users/hobu/:/data pdal/pdal:1.5 ^
       pdal translate /data/input.las /data/output.pcd

[GIS] Importing LiDAR (.las) data in PostgreSQL with PDAL

As HowardButler stated you need to use the JSON option for any PDAL version over 1.5. I am using PDAL version 1.7.2 and PostGreSQL version 9.6.

1) Create a text file using NotePad as described below. This particular JSON script reads from a standard .LAS file and writes to a PostGreSQL database table. The PostGreSQL database has the pointcloud and postgis extensions enabled. The lidar records go in as patches (groups) of 600 per database record. I don't know why they call them "patches" but that's what they decided to name them. Note the double \ in the filename path. JSON doesn't like single backslashes so you need to escape them. Save the file as theNameOfYourJsonFile.TXT. Obviously you will change the SRID to whatever is appropriate for what you are working on.

{
  "pipeline":[
    {
      "type":"readers.las",
      "filename":"C:\\foldername\\foldername\\foldername\\nameOfLasFile.las",
      "spatialreference":"EPSG:26916"
    },
    {
      "type":"filters.chipper",
      "capacity":600
    },
    {
      "type":"writers.pgpointcloud",
      "connection":"host='localhost' dbname='nameOfYourDb' user='yourUserName' password='yourPassword'",
      "table":"pcpatches",
      "compression":"dimensional",
      "srid":"26917"
    }
  ]
}

2) I am using the Windows version of PDAL that came with OSGeo4W so in my case I then open the command window to run the next step. If you are using a different operating system you may need to do this next step differently. At the c: prompt in the command window type in the following:

         pdal pipeline -i C:\foldername\theNameOfYourJsonFile.txt

The pdal pipeline command will run using the JSON from the text file you made. JUST LET IT RUN! It might take anywhere from a few minutes to 10 or 15 minutes. Remember point clouds are HUGE! When complete you will have a PostGreSQL table with your lidar records imported. Each record will contain groups of 600 points that can be extracted using SQL. Here is one way to view the data, there are many other ways available:

SELECT PC_AsText(
PC_Explode(nameOfFieldThatContainsYourPatches))
FROM tableName;

Best Answer

Related Solutions

[GIS] Converting .las to .pcd file using PDAL

[GIS] Importing LiDAR (.las) data in PostgreSQL with PDAL

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