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I need to merge ~8k of .asc files into one big one. To convert each one into a raster first would take forever .. is there a possibility to convert multiple ascii files to raster in ArcGIS?

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    Can't you use these "as is" without having to convert? – Jakub Sisak GeoGraphics Sep 2 '15 at 14:43
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    Doing anything with 8000+ ASCII files is going to take forever. Your question is unclear -- Please specify what you mean by "merge" and "convert multiple". – Vince Sep 2 '15 at 14:57
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    ArcGIS is not the right tool for this amount of data. Try gdal_translate to convert your ascii to e.g. GeoTiff and the gdal_merge.py to merge the raster files to one raster image – Iris Sep 2 '15 at 15:13
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ASCII is an Esri format, you don't need to convert it first. That is assuming the files called .asc are really Esri ASCII Grid format and not just some other text raster format (like X,Y,Z).

Open one in Notepad++ (or your other favorite text editor) and see that it conforms to this format:

NCOLS xxx
NROWS xxx
XLLCENTER xxx | XLLCORNER xxx
YLLCENTER xxx | YLLCORNER xxx
CELLSIZE xxx
NODATA_VALUE xxx
row 1
row 2
...
row n

Now assuming that it does, create a File Geodatabase and then in it create a Mosaic Dataset (you indicated ArcGis 10.3, this is new at 10.0) by right clicking in the database and select on the context menu new::Mosaic Dataset, fill in the blanks and hit OK.

Now you have an empty dataset right click and select add rasters then select all your .asc files. This does not convert your files, only linking, this operation should be fairly fast.

For performance it's a good idea to build pyramids; only the links and pyramids are stored in the database, the rasters are unchanged (so don't move or delete them). You can use the mosaic dataset 'as is' in ArcMap or using a tool like Copy Raster you can export it to a single image (like Img or GeoTiff/BigTiff).

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