I’ve been preparing a tilecache of UK Ordnance Survey mapping for a couple of big projects, combining Open Data with licensed Landranger (1:50,000) and Explorer (1:25,000) mapping. The tilecache utility consumes and generates its cache from a WMS of the mapping, and fir this I’ve done most of the work using MapServer. However I’ve hit some stubborn problems with a few areas of mapping, for example this area where I have a blank strip enclosing a tile boundary. The original tiles are fine, but for some reason when render by MapServer we get this blank area.

In another couple of areas, the WMS requests to MapServer simply failed with this error sequence:

[Sat Jan 21 09:30:02 2012].785000 msDrawRasterLayerLow(OS50KColourRaster): entering.
[Sat Jan 21 09:30:02 2012].791000 msResampleGDALToMap in effect: cellsize = 5.000000
[Sat Jan 21 09:30:02 2012].792000 msDrawGDAL(OS50KColourRaster): using RAW_WINDOW=3509 0 491 652, dst=0,0,491,652
[Sat Jan 21 09:30:02 2012].792000 msDrawGDAL(): red,green,blue,alpha bands = 1,0,0,0
[Sat Jan 21 09:30:02 2012].891000 msResampleGDALToMap in effect: cellsize = 5.000000
[Sat Jan 21 09:30:02 2012].892000 msDrawGDAL(OS50KColourRaster): using RAW_WINDOW=3509 3348 491 652, dst=0,0,491,652
[Sat Jan 21 09:30:02 2012].892000 msDrawGDAL(): red,green,blue,alpha bands = 1,0,0,0
[Sat Jan 21 09:30:02 2012].998000 msResampleGDALToMap in effect: cellsize = 5.000000
[Sat Jan 21 09:30:02 2012].998000 msDrawGDAL(OS50KColourRaster): using RAW_WINDOW=0 0 812 652, dst=0,0,812,652
[Sat Jan 21 09:30:02 2012].998000 msDrawGDAL(): red,green,blue,alpha bands = 1,0,0,0
[Sat Jan 21 09:30:03 2012].122000 msResampleGDALToMap in effect: cellsize = 5.000000
[Sat Jan 21 09:30:03 2012].122000 msDrawGDAL(OS50KColourRaster): using RAW_WINDOW=0 3348 812 652, dst=0,0,812,652
[Sat Jan 21 09:30:03 2012].122000 msDrawGDAL(): red,green,blue,alpha bands = 1,0,0,0
[Sat Jan 21 09:30:03 2012].148000 GetBlockRef failed at X block offset 0, Y block offset 499: Unable to access file. GDALDatasetRasterIO() failed: drawGDAL()
[Sat Jan 21 09:30:03 2012].149000 msDrawMap(): Image handling error. Failed to draw layer named 'OS50KColourRaster'.
[Sat Jan 21 09:30:03 2012].149000 freeLayer(): freeing layer at 024265B0.

again the underlying images were apparently fine.

And of course this has left gaps in the tile cache.

So, in order to overcome these issues I am setting up the WMS using GeoServer to see if that can fill the gaps.

My starting point is a folder containing the TIFF images, each with a .TFW world file.

Gotchas:

There are instructions for setting up a raster WMS in GeoServer here, but here is my summary with details of where things can go wrong:

• Create a workspace, in my case “ESDM_UK_BaseMaps” with a namespace of “https://www.ordnancesurvey.co.uk/”
• Create a store – this will equate with one type of map, in this case the Landranger 1:50,000 colour raster.
• Choose ImageMosaic as the type of store
• Select the workspace, and give the store a name, in this case “OS50K”
• For the connection parameters, enter the folder that contains the images, in my case “file:D:\MapData\OrdnanceSurvey\50KRasterColour”
• GeoServer will then automatically scan the folder and create a shapefile index for the tiles (or “granules” as they bizarrely call them). At this point I was confronted with this error message “Could not list layers for this store, an error occurred retrieving them: Unable to acquire a reader for this coverage with format: ImageMosaic”.

This is a commonly observed error by all accounts with lots of fairly grumpy exchanges among GeoServer users and developers about how to reproduce it and what might be the causes. It seems it can be raised by a raft of different issues. So I returned to a close reading of the instructions and also found Adrian Walker’s useful blog post about configuring OS maps in GeoServer.

It turns out I had two problems:

First the .TFW extension must be lower case. So in a command prompt run

ren *.TFW *.tfw

(note that the image file extension does not have to be lower case)

Second, GeoServer needs a .prj file for every image (not required by MapServer).

These can be generated using a python script (thanks to Adrian Walker again for this tip):

import glob
content = 'PROJCS["OSGB 1936 / British National Grid", GEOGCS["OSGB 1936", DATUM["OSGB_1936", SPHEROID["Airy 1830",6377563.396,299.3249646, AUTHORITY["EPSG","7001"]], AUTHORITY["EPSG","6277"]], PRIMEM["Greenwich",0, AUTHORITY["EPSG","8901"]], UNIT["degree",0.01745329251994328, AUTHORITY["EPSG","9122"]], AUTHORITY["EPSG","4277"]], UNIT["metre",1, AUTHORITY["EPSG","9001"]], PROJECTION["Transverse_Mercator"], PARAMETER["latitude_of_origin",49], PARAMETER["central_meridian",-2], PARAMETER["scale_factor",0.9996012717], PARAMETER["false_easting",400000], PARAMETER["false_northing",-100000], AUTHORITY["EPSG","27700"], AXIS["Easting",EAST], AXIS["Northing",NORTH]]'
tifs = glob.glob('*.tif') 
for tif in tifs:  
    prj = tif.split('.')[0] + '.prj'  
    file = open(prj,'w')  
    file.writelines(content)  
    file.close()

After resolving these issues, it was necessary to return to step 2 above to create the store, but I got the error again! OK, GeoServer had created the index shapefile already, and it was bad due to the missing prj and tfw files; unfortunately GeoServer will not overwrite this file so I had to manually delete the files (the shapefile is named after the store, and comprises seven files with various extensions).  After that, creating the store worked fine.

Hint: if you examine the .prj file in a text editor and it looks like this, then you need to bin it and start again:

LOCAL_CS["Generic cartesian 2D",
LOCAL_DATUM["Unknow", 0],
UNIT["m", 1.0],
AXIS["x", EAST],
AXIS["y", NORTH]]

So on clicking “Save” for the new store, it showed a twirly for a couple of minutes while creating the index shapefile, and then transfers you to the New Layer page, showing the new layer that can be published (it names the layer after the folder containing the images). From there it is a simple matter to complete the configuration of the layer.

Note: I think I could have configured a single store one folder higher up, and this would have included each subfolder of TIFFs as a separate layer.

Outcome

Once I had finally got my GeoServer WMS up and running it was time to see whether could fills the gaps in my tilecache…. and I’m very pleased to say yes it could!

I’ve posted separately about performance and image quality from the two servers.