Georeferencing PNG from bounding box in GDAL

gdalgeoreferencingpng

I have an image in PNG format and bounding box coordinates (upper left, lower right). How do I georeference it using command-line interface tools such as GDAL (or Python if it is not possible)?

In the end, I want to open the PNG image in QGIS but NOT use QGIS to georeference it (because I need to create an automation script).

for instance, the image name is image.png and the bounding box is 53.1111876266,11.7303748998,55.5611387985,13.1036943879. What should I type in my terminal?

Best Answer

gdal_translate is the tool. the command below will take the image.png, assign/georeference based on the bounding box (upper left, lower right) coordinates, and create an .aux.xml which containes the georeferencing location. Use the -a_ullr attributes. Further, read the documentation.

C:/> gdal_translate image.png output_georeferenced.png -a_ullr 53.1111876266 11.7303748998 55.5611387985 13.1036943879 -a_srs EPSG:4326

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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] Create a World File from bounding box

Assuming a zero-based rotation factor for x and y, here you need the png file image dimensions in order to derive x and y pixel size.

Rudimentary example:

import xml.etree.ElementTree as etree

image_width = 500
image_height = 300
x_rotation = 0.0
y_rotation = 0.0

bbox_xml = etree.fromstring('''<LatLonBox>
<north>30.46370000000000</north>
<south>25.15370000000000</south>
<east>89.06550000000000</east>
<west>83.06550000000000</west>
</LatLonBox>''')

minx = float(bbox_xml.find('west').text)
miny = float(bbox_xml.find('south').text)
maxx = float(bbox_xml.find('east').text)
maxy = float(bbox_xml.find('north').text)

x_pixel_size = (maxx - minx) / image_width
y_pixel_size = ((maxy - miny) / image_height) * -1

print(x_pixel_size)
print(y_rotation)
print(x_rotation)
print(y_pixel_size)
print(minx)
print(miny)

Best Answer

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[GIS] Georeferencing png using gdal

[GIS] Create a World File from bounding box

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