[GIS] Technique using GDAL to overlay / fit one PNG map to directly overlay on another

gdalgdalwarpoverlay

I have one .PNG basemap with known North edge of Latitude = 52.48, a South edge of Latitude = 21.94, an east edge of Longitude = -61.87 and a west edge of Longitude = -129.37.

And I have another PNG image overlay with a known North edge of Latitude = 50.03, a South edge of Latitude = 22.02, an east edge of Longitude = -67.24 and a west edge of Longitude = -124.99.

Is there a way using GDAL or other simple scripting tools to process the second PNG image to be in an output that directly overlays (same x/y dimensions) the basemap?

I have no trouble generating map tiles that overlay each other beautifully – but I seem to be having no luck figuring out the syntax of the seemingly simpler case.

Help in the form of a simple example is much appreciated!

To be clear: the goal is an output .png file that has the exact same x/y dimensions as the basemap, and which can directly overlay on top of it.

Best Answer

EDITED after feedback from question poster.

Step 1: if files are not georeferenced, try the methods described here, Georeferencing using GDAL?. Also world files could work for weither png or tif (http://en.wikipedia.org/wiki/World_file; http://www.gdal.org/gdal_translate.html).

Step 2: two methods could work for the clipping. Trying gdaltranslate with the following option -projwin ulx uly lrx lry: would work if you have the coordinates as the question seems to imply.

OR, you have the option of using gdaltindex newclip.shp basemap.png to create a clip shapefile. The shapefile can be used in the following way

gdalwarp -cutline /path/to/newclip.shp -crop-to-cutline oldimage.tif newimage.tif

remember that the path I show is for linux. Windows would be C:\something\clip.shp. Also, the example does not account for CRS. There is the option of replacing the cutline with -te xmin ymin xmax ymax

helpful link: http://www.gdal.org/gdalwarp.html

hope this helps :)

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

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

[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.

Best Answer

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