Loss of precision when importing LAS files with PDAL

Question

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?

Answer 1

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

Answer 2

Bounding boxes are stored as double in a LAS files. But the point cloud coordinates are stored as integer and converted to double with a scale factor and an offset. It means that the point cloud coordinates are stored with a given accuracy (typically 2 decimals) but the bounding box could not conform to the standard and be stored with an irrelevant accuracy that does not match with the point cloud. I think your file does not conform to ASPRS standard but I may be wrong without seeing the file.

Anyway as mentioned in comments your accuracy is subatomic

Answer 3

The problem is that PDAL is not conserving the previous offset, the default value for offset is 0 (source: PDAL las.writers page). This end up in it trying to save untranslated coordinates, which force it to drop some some significant values as the coordinates are too big. To correct it, you need to make PDAL offset the pointcloud when saving it.

If in command line, add :

--writers.las.offset_x=auto --writers.las.offset_y=auto --writers.las.offset_z=auto

if json pipeline, add to the writers.las :

"offset_x":"auto",
"offset_y":"auto",
"offset_z":"auto",