[GIS] How to create a 4096×4096 png from a 1440×720 data set using GDAL

gdalgoogle mapsnetcdf

I'm having a bear of a time trying to do something that seems fairly straightforward. I'm trying to take a netcdf file from ftp://eclipse.ncdc.noaa.gov/pub/OI-daily-v2/NetCDF/2011/AVHRR-AMSR/ and turn it into a 4096×4096 png so that I can tile it up for use w/google maps. I've been using the following commands but it always seems to create blank areas on the east and west borders:

/usr/local/bin/gdal_translate -a_srs EPSG:4326 -outsize 4096 4096 -a_ullr 0 90 360 -90 NETCDF:"input.nc":sst -of netCDF output.nc
/usr/local/bin/gdaldem color-relief output.nc -b 1 -alpha colorscale.txt -of PNG output.png
/usr/bin/convert output.png -transparent #FFFFFF output.png

Am I just approaching this the wrong way? Am I using the wrong tools?

FWIW – gdal_translate is v1.8.0

Thanks in advance.

UPDATE: "Answer" below from Dija – colleague of mine – is actually a response to the comments on the question itself.

UPDATE Accidentally added an answer instead of a comment. Moving it here.

I thought I'd weigh in here. I've been working on this issue for a little while now. I've tried many iterations of the process. I scaled these images to the 4096×4096 as it actually creates a better image than if not. Meaning, if I leave the image in it's native size, 1440×720 I get a larger gap at the prime meridian (for this data set) and at the 180th meridian for other data sets. I scaled the output image to be a multiple of 256 (to fit better with google) and get various results.

If I scale it to 1536×768 the gap shows up to the right of the 0 meridian. If I scale to 1280×640 it's to the left. If I leave it alone, it's pretty much splits it.

I have taken the base image that gdaldem spits out, cut it in half and matched the edges up…they line up perfectly. This leads me to think that there's something weird going on with gdal2tiles.py

I wrote my own tiler that I'm getting ready to turn loose on our images to see if the gap remains.

I'll put together some screen shots to show what I am talking about with how the gap as well as report on what my tiler does.

Thanks for any input you might have. It's appreciated.

UPDATE 2 I ran the maptiler I wrote against the original raster and it works fine with out a gap. gdal2tiles.py seems to be the problem. It introduces a transparent edge to the last tile that touches the 180th meridian.

Best Answer

gdal_grid has parameters that let you designate the extent coordinates of your output raster.

gdal_grid -txe [xcoord1] [xcoord2] -tye [ycoord1] [ycoord2]

where the lower left coordinate would be (xcoord1,ycoord1) and the upper right would be (xcoord2,ycoord2)

GDAL will interpolate your data according to your output extents

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[GIS] Georeferencing png using gdal

Here's a world file for you (as per Gene's suggestion). Cut and paste the following into a text editor and name the file the same as your full-sized map but give it the extension '.pgw' instead on '.png':

1.0
0.0
0.0
-1.0
0.0
3231.0

I've made the following assumptions:

Your map has a resolution (pixel size) of 1m square (i.e. each pixel is a reasonable stride length for a fantasy game humanoid character). If I am wrong, edit lines 1 and 4 accordingly.
I have assumed that your map is not rotated.
I have assumed that your LL corner is at 0,0 in your world. If your pixel size is bigger than 1m square then you will need to multiply your Y coordinate (3231.0) accordingly.
Technically the UL coordinate (last two lines) is the center of your pixel. I have been lazy and set it to the corner. This in theory means that your coordinates would be consistently half a meter out but it's a fantasy world so nobody will know; it is consistent so won't screw up any distant calculations; and it saves a negative value as one of your origin coordinates. Adjust if you are sufficiently worried :)

I would then re-tile your map from the georeferenced full-sized one (this saves you making a world file for each tile)

Now, if you load your fantasy map into a GIS or similar system, you will probably be asked for the Spatial Reference System (SRS). The World file doesn't encapsulate that. It just locates and orients the image file in space relative to a nominal 0,0 origin. Since this is a fantasy map, all you need to do is pretend that it is in the real world somewhere and pick any coordinate system that gives you a flat projection with units in meters (or feet if you prefer but you will may have to adjust the values for resolution in the world file). A couple of suggestions for SRS could be:

EPSG: 3857 (aka 900913)
EPSG: 27700 (UK National Grid - the advantage of a national grid system UK or otherwise, is that you mostly stay clear of the poles and the international dateline and can hopefully avoid any weirdness which may result from such a location)

Your choice might not matter at all... on the other hand, you choice of SRS might be influenced by what you intend to do with your KML/CSV file (like you might be making a game with a post-apocalyptic theme but set in Quebec ...with zombies...). If you are using Google Earth or similar as a 'platform' for your game/fantasy world, so long as all your other data are in the same coordinate system, all should be well.

However, I do wonder if you may be better sticking with your PNG file as a text format like KML/CSV will be big. A PNG may be better for an internet-based game (reducing bandwidth). If you have a stand-alone scenario, most programming languages understand image formats well (especially if you call the GDAL libraries) and so I'm not convinced that a text-based format is even necessary here - but that's your choice of course.

[GIS] GDAL C++ API: How to create PNG or JPEG from scratch

As user30184 said, in Python, the process would be creating a memory raster of the same dimensions (layers and layer extension), and executing the CreateCopy after that:

driver = gdal.GetDriverByName( 'MEM' )
driver2 = gdal.GetDriverByName( 'PNG' )
ds = driver.Create( '', 255, 255, 1, gdal.GDT_Int32)
ds2 = driver.CreateCopy('/tmp/out.png', ds, 0)

Then, just work with ds2

This should be translated to C++, of course. You can take a look at the Using CreateCopy() section in the tutorial: http://www.gdal.org/gdal_tutorial.html

Best Answer

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[GIS] Georeferencing png using gdal

[GIS] GDAL C++ API: How to create PNG or JPEG from scratch

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