问题
I have a flat 400x400 pixels image which represents an area of 400x400 kilometers. I need to draw this image as an overlay on my OpenLayers mapping system. I use OpenStreetMap/Google Maps as base layers. I want to draw the overlay image using an Image Layer and I want to center my overlay image at a specific lat/lon point. The projection of the base layer is EPSG:900913 which is, in fact, the projection used by OpenStreetMap and Google Maps.
What I have understood (please correct me if I'm wrong), is that I can't use a flat, 'unprojected' image as an Image Layer, but I have to do a sort of 'reprojecting' to adjust the image to be compliant with the projection of the map, which is EPSG:900913. Searching here and the net, it seems I should use a library/tool called GDAL to do this, but whereas I am completely new to this I ask you:
- Can anyone address me on how to use it?
- Could you possibly tell me if what I have just said is correct and the way I described here is valid?
- How could I use gdal to translate my flat unprojected 400x400 pixels image to an image projected in EPSG:900913, and suitable to be put as an Image Layer as an overlay?
Thanks.
I added the image as requested.
The answer provided by @capdragon was useful and let me going one step further. Anyway I have still some problems. I did this:
- used gdal_translate to set GCPs to the four corners and the center of the image like this:
gdal_translate -of "GTIFF" -gcp 0 0 8.6923 45.5427 -gcp 400 0 13.8149 45.5427 -gcp 200 200 11.2536 43.771702 -gcp 0 400 8.8413 41.9460 -gcp 400 400 13.6659 41.9460 quadrprod.png quadrprod_gpc.tif
This way I get a quadrprod_gcp.tif output. Then.
- used gdalwarp to apply the GCPs set in the point above and set the right projection, which is EPSG:4326 like this:
gdalwarp -t_srs EPSG:4326 quadrprod_gcp.tif quadrprod_gcp_warped.tif
This way I get a final quadrprod_gcp_warped.tif. The resulting image is no more 400x400 pixels but is 458x331 pixels. Then if I use gdalinfo to look at it, I found this:
gdalinfo quadrprod_gcp_warped.tif
Driver: GTiff/GeoTIFF
Files: quadrprod_gcp_warped.tif
Size is 458, 331
Coordinate System is:
GEOGCS["WGS 84",
DATUM["WGS_1984",
SPHEROID["WGS 84",6378137,298.257223563,
AUTHORITY["EPSG","7030"]],
AUTHORITY["EPSG","6326"]],
PRIMEM["Greenwich",0],
UNIT["degree",0.0174532925199433],
AUTHORITY["EPSG","4326"]]
Origin = (8.768046497499189,45.547291862003739)
Pixel Size = (0.010877474938832,-0.010877474938832)
Metadata:
AREA_OR_POINT=Area
Image Structure Metadata:
INTERLEAVE=PIXEL
Corner Coordinates:
Upper Left ( 8.7680465, 45.5472919) ( 8d46'4.97"E, 45d32'50.25"N)
Lower Left ( 8.7680465, 41.9468477) ( 8d46'4.97"E, 41d56'48.65"N)
Upper Right ( 13.7499300, 45.5472919) ( 13d44'59.75"E, 45d32'50.25"N)
Lower Right ( 13.7499300, 41.9468477) ( 13d44'59.75"E, 41d56'48.65"N)
Center ( 11.2589883, 43.7470698) ( 11d15'32.36"E, 43d44'49.45"N)
Band 1 Block=458x4 Type=Byte, ColorInterp=Red
Mask Flags: PER_DATASET ALPHA
Band 2 Block=458x4 Type=Byte, ColorInterp=Green
Mask Flags: PER_DATASET ALPHA
Band 3 Block=458x4 Type=Byte, ColorInterp=Blue
Mask Flags: PER_DATASET ALPHA
Band 4 Block=458x4 Type=Byte, ColorInterp=Alpha
As you can see the 4 corners and the center lat/lon are different from the values I set using -gcp values. My questions:
- Why the lat/lon values returned by gdalinfo are different from the values I set in step 1 using -gpc?
- Is it normal or am I doing something wrong?
- What is next step? Should I translate the image again from EPSG:4326 to EPSG:900913?
- When I place my image as overlay in openlayers, when I create the bounds object, should I use as southwest and northeast corners, the values I used in the -gcp or the final values I see with gdalinfo?
回答1:
What you're saying is correct.
You basically want to set some control points (reference points) using gdal_translate then warp the image using gdalwarp.
So for a rough projection you could simply use the corner cordinates to set 4 control points like so. (These examples are using EPSG:4326, but you can use EPSG:900913.
Set 4 control points (corner coordinates)
gdal_translate -a_srs EPSG:4326 -gcp 0 0 -89.38939600 30.39282800 -gcp 1024 0 -87.00029400 30.01043900 -gcp 0 1250 -89.99424800 27.37030800 -gcp 1024 1250 -87.67748400 26.98606100 "originalImage.tif" "image_trans.tiff"
The above is saying that pixel x=0 and y=0 is equal to "-89.38939600 30.39282800" pixel x=1024 and y=0 is -87.00029400 30.01043900 and so forth.
Warp the image:
gdalwarp -dstalpha -t_srs EPSG:4326 image_trans.tiff image_warped.tiff
If the warping isn't good enough you're going to need to set more control points on the image.
UPDATE
I downloaded your image and ran the commands you have. Everything worked fine for me: |
captDragon@liberatecuba:/media/Data/tmp/orthoTesting$ gdal_translate -of "GTIFF" -gcp 0 0 8.6923 45.5427 -gcp 400 0 13.8149 45.5427 -gcp 200 200 11.2536 43.771702 -gcp 0 400 8.8413 41.9460 -gcp 400 400 13.6659 41.9460 quadrprod.png quadrprod_gpc.tif
Input file size is 400, 400
0...10...20...30...40...50...60...70...80...90...100 - done.
captDragon@liberatecuba:/media/Data/tmp/orthoTesting$ gdalinfo quadrprod_gpc.tif
Driver: GTiff/GeoTIFF
Files: quadrprod_gpc.tif
Size is 400, 400
Coordinate System is `'
GCP Projection =
GCP[ 0]: Id=1, Info=
(0,0) -> (8.6923,45.5427,0)
GCP[ 1]: Id=2, Info=
(400,0) -> (13.8149,45.5427,0)
GCP[ 2]: Id=3, Info=
(200,200) -> (11.2536,43.771702,0)
GCP[ 3]: Id=4, Info=
(0,400) -> (8.8413,41.946,0)
GCP[ 4]: Id=5, Info=
(400,400) -> (13.6659,41.946,0)
Image Structure Metadata:
INTERLEAVE=PIXEL
Corner Coordinates:
Upper Left ( 0.0, 0.0)
Lower Left ( 0.0, 400.0)
Upper Right ( 400.0, 0.0)
Lower Right ( 400.0, 400.0)
Center ( 200.0, 200.0)
Band 1 Block=400x5 Type=Byte, ColorInterp=Red
Mask Flags: PER_DATASET ALPHA
Band 2 Block=400x5 Type=Byte, ColorInterp=Green
Mask Flags: PER_DATASET ALPHA
Band 3 Block=400x5 Type=Byte, ColorInterp=Blue
Mask Flags: PER_DATASET ALPHA
Band 4 Block=400x5 Type=Byte, ColorInterp=Alpha
来源：https://stackoverflow.com/questions/18960229/how-to-project-a-flat-image-to-spherical-mercator-epgs900013-to-use-in-a-image