[OSGeo Africa] Compression of Aerial Imagery

[Sibonelo Dlamini]

Mr Parker

I used RGB in my space. There is something called Mrsid.wig format

Please forward me your s

Regardless
Sibonelo
GtgGISc0817

On Fri, 12 Jul 2024, 18:35 Grant Slater via Africa, <africa at lists.osgeo.org>
wrote:

> Hi Aslam,
>
> Answers below...
>
> On Thu, 11 Jul 2024 at 14:20, Aslam Parker via Africa
> <africa at lists.osgeo.org> wrote:
> >
> > Good day
> >
> > Can anyone please advise on recommended compression for 3 Band (RGB) 16
> bit aerial imagery that is suitable in a photogrammetric workflow?
> >
> > Also, is there a compression that can be recommended for 4 band 8 bit
> RGBNIR imagery that can be used in photogrammetric workflow. Although CD:
> NGI acquires 4 band imagery, it only orthorectifies 3 bands , due to the
> limitation of JPEG compression, which can only compress three bands.
> >
>
> The simple answer is: It depends. There are Saving Performance / Image
> Quality / File Size / $$$ cost tradeoffs.
>
> Do you perhaps have an example of the workflow steps? Ideally with
> file load / save steps, because each additional save step is likely to
> increase compression artifacts.
> Could publish samples of the files in original format?
>
> Some initial thoughts...
>
> * ECW: is very common in the proprietary world, but I understand it
> has expensive licensing costs. I have no experience with it.
> * MrSID: as above.
> * JPEG (GeoTIFF): is open, widely adopted, quality / file size is
> poor. 12-bit not widely adopted/supported. YCBCR "RGB" only.
> * LZW, DEFLATE (GeoTIFF): are open, lossless, but huge file size.
> Likely option for intermediate steps.
> * ZSTD (GeoTIFF): open, lossless, but large file size. Likely option
> for intermediate steps.
> * LERC / LERC_DEFLATE / LERC_ZSTD (ESRI) (GeoTIFF): open, lossy /
> lossless, limited adoption.
> * JPEG2000  (JP2) is open, but software support is poor / mixed
> adoption / buggy.
> * JPEG-XL / JXL (GeoTIFF) is open, but has very limited adoption / buggy.
> * WEBP (GeoTIFF) is very good quality for small file size, Slow to
> compress. What I use for aerial.openstreetmap.org.za
>
> Outside of compression:
>
> * Files should be internally tiled. For GeoTIFF this needs to be
> specified when saving.
> * Published / Archived files should not contain internal "overviews".
> This bloats the filesize by 20%+.  gdaladdo, QGIS or similar tools can
> create external overviews if needed.
> * Ideally RGB and NIR are published as separate files.
>
> Observations about the 25cm series that are being published:
>
> * Publish 1 file per grid square. Do not combine squares: eg:
> 3217DD_23_3317BB_03_2022_1556_RGB_RECT.tif and
> 2816DA_03_BC_23_2022_1584_RGB_RECT.tif
> The combined squares makes it more difficult for end users to find
> gaps and find missing files.
> Publish a list of grid squares which will / are available.
> Sometimes small bits get missed / not published:
> eg: 2420CC_01 was missed in 2022 Capture: 1669.
> eg: 2523AC_04 and 2523AC_05 were missed in 2021 Capture: 1447
>
> * Multiple rounds of Compression / Sensor issues?
> Quality seems to vary greatly. Output files are JPEG at Q95 or Q97 but
> it seems like during workflow the imagery has been over compressed and
> has severe compression artifacts.
> eg: 2729DD_05_2022_1712_RGB_RECT.tif (poor) vs
> 2730CC_01_2022_1713_RGB_RECT.tif (ok)
> Example view:
> https://aerial.openstreetmap.org.za/#zoom=19&lat=-27.754098&lon=30.001725
>
> * High number of published files are corrupt or truncated and
> therefore are unusable.
> eg: 3220AD_17_2022_1625_RGB_RECT.tif and 3325CB_24_2021_1437_RGB_RECT.tif
>
> * High number of published files use a different projection than the
> other files in the same 50k grid square.
> eg: 2624CC_06_2021_1455_RGB_RECT.tif and 2520AC_01_2022_1669_RGB_RECT.tif
>
> The imagery published by NGI is an amazing resource. Thank you!
> Next 5 year cycle 20cm? ;-)
>
> Kind regards,
>
> Grant
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