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--- stitching_etopos [2023/07/08 14:03] – bushwalking
+++ stitching_etopos [2023/08/31 08:55] (current) – Add link bushwalking
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 Note that it is skewed towards Windows users, and assumes a certain level of technical knowledge (eg running commands on the command line), and that you have GDAL installed and the path to the binaries in your PATH.
+The following answer from GIS Stack Exchange has more details on resampling and compression options:
+https://gis.stackexchange.com/questions/464734/convert-geopdf-to-8-bit-geotiff-using-gdalwarp/464842#464842
 ===== 0. Background on UTM and map dimensions =====
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 ===== 1. Download eTopos =====
+These can be downloaded from [[https://portal.spatial.nsw.gov.au/portal/apps/webappviewer/index.html?id=06e3c2e0de1e4efda863854048c613c6|SIX Maps]]. Currently the 2017 versions.
 While there is a 5 map per day limit, this can be naughtily bypassed by grabbing a copy of the {{ :20170212_nsw_map_boundaries.xlsx |Excel spreadsheet}} on the [[downloads|Downloads]] page, which has direct links to all of the topos. You can drop a bunch of links into your favourite downloader, or just click on them one by one.
 While you're there, also grab the {{ :20160506_nsw_map_bounds.geojson |GeoJSON version of the NSW Map Boundaries}}, as you'll need a copy of it in the next stages.
+There is also a [[https://portal.spatial.nsw.gov.au/portal/apps/webappviewer/index.html?id=06e3c2e0de1e4efda863854048c613c6|newer site]] that has a range of versions from 2011 to 2022.
 ===== 2. Remove borders =====
 Once you have the PDFs, the next step is to strip off the border of the PDF and convert to a raster format. We can do this for a single image using gdalwarp, by setting a few options, including:
-  * -t_srs <target spatial reference> - I've used EPSG:28356, which is MGA94 Zone 56 (eastern NSW). All maps need to be in the same co-ordinate system in order to merge them in the next step.
+  * -t_srs <target spatial reference> - I've used EPSG:28356, which is MGA94 Zone 56 (eastern NSW). All maps need to be in the same co-ordinate system in order to merge them in the next step. This may not be needed(?)
   * -r <resampling_method> - cubic seems to be the recommended option given that we aren't trying to resize
   * -cutline - applies a mask to the raster (confusingly, this doesn't actually crop the image - you also need "crop_to_cutline")
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   * -crop_to_cutline - crops to the mask applied under "cutline"
   * -dstalpha - sets an output alpha band so that the pixels that are outside the cutline but inside the bounding box are transparent. This is needed when merging in the next step
+  * -co COMPRESS=LZW (optional) - will reduce the size of the intermediate GeoTiffs
 Sample command in full - you'll need to set your own directories, both for input and output file, and for the location of the NSW map boundaries GeoJSON:
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 The GeoTiff file can be used as is in many applications.
+There are other compression options (''-co COMPRESS=JPEG'', ''-co COMPRESS=WEBP'') that will result in a smaller file, but may limit where the file can subsequently be used.
+The ''-co tiled=YES'' option may be useful for improving the speed of the resulting GeoTiff.
 ===== 5. Create OZFX4 file for OziExplorer =====
-For OziExplorer, the TIF can be used directly (use File->Import Map->Single DRG Map - and you will need the GeoTiff support library DLL), but it is faster and more space-efficient if converted to OZFX4 format. This can be done using Img2Ozf, downloadable from the OziExplorer website. Default settings are reasonable. As an example, a 4GB GeoTiff was converted to a 400MB OZFX4 file - a 90% reduction in size.
+For OziExplorer, the TIF can be used directly (use File->Import Map->Single DRG Map - and you will need the GeoTiff support library DLL), but it is faster and more space-efficient if converted to OZFX4 format. This can be done using [[https://www.oziexplorer4.com/img2ozf/img2ozf.html|Img2Ozf]], downloadable from the OziExplorer website. Default settings are reasonable. As an example, a 4GB GeoTiff was converted to a 400MB OZFX4 file - a 90% reduction in size.