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Home - General / All posts - Excessive Image size from imported ASCII grid files
sknox59 post(s)
#25-Feb-21 17:00

So I have this raster dataset OS Terrain 50 which is 153MB, zipped, on disk.

I import a subset of these ASC into Manifold, importing about 50 of these 10km x 10km tiles, each around 191KB on disk. I then merge these tiles into a single image.

I end up with an image size of 140,000 x 160,000 (this is the rectangle of my AOI but I don't need all the data within that rectangle, 386,103 records, and my Manifold project balloons from about 3GB to 36.5 GB.

Am I missing something here? This seems disproportionate. Am I doing something wrong? I don't see how a small subset of a zipped 153MB dataset can balloon by 30GB. Appreciate that I might have crated some artifacts during the merge, and that potentially storage costs in the sparse matrix are not nill, but this is way beyond what I was expecting.

Is there a more efficient way to undertake what I am trying to do here - get a single, irregularly shaped large, tiled image by combining smaller images?

Using Manifold 9.0.173.2

sknox59 post(s)
#25-Feb-21 17:38

The problem seems to be, when merging adjacent images, the tile count goes from 4 x 2 to 12403. Resampling being done somewhere. All the tiles are in the same projection.

tjhb

9,627 post(s)
#25-Feb-21 17:44

Are using source metrics, or default metrics (1 unit per pixel)?

You might want to check your Merge workflow.

sknox59 post(s)
#25-Feb-21 17:50

That would explain it, thanks. Must RTFM.

tjhb

9,627 post(s)
#25-Feb-21 18:21

It's not necessarily obvious.

See this thread.

Shortcut: right-click one of the images in the Merge list, and select Use Coordinate System.

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