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I have been trying to convert windspeed files to rasters without success. It was discussed in an Arcgis forum which I followed. This involved replacing semi-colons with spaces and adding a header.

I was hoping to include one of the original .asc files together with a .csv version I 'cleaned up'. Unfortunately I cannot see how to do this without copying and pasting the text. The first line of the original .asc file is:

Conversion of speed45i.dat   Wind m/s  Wind Speed @ 45m N. Ire 
(100,  499); 0.0; 0.0; 0.0; 0.0; 0.0; 0.0; 0.0; 0.0; 0.0; 0.0; 0.0; 0.0; 0.0; 0.0; 0.0; 0.0; 0.0; 0.0; 0.0; 0.0; 0.0; 0.0; 0.0; 

I changed this to:

ncols 700
nrows 1300
x||corner 0
y||corner 0
cellsize 1000                                                                                               "
"( 0 1299) 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 ;"

When I usen the raster translate convert format plug-in I got the error message:

The process failed to start. Either the invoked program is missing, or you may have insufficient permissions to invoke the program.

Any suggestions gratefully received.

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you could always try putting an example in DropBox and share a link, if you want more specific help with the particular dataset. –  RyanDalton Jan 9 '13 at 19:27
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2 Answers

The NoData value is missing in your ascii file and you have x|| and y|| instead of xll and yll. I am assuming that your NoData is -999.

try this:

NCOLS 700
NROWS 1300
XLLCORNER 0
YLLCORNER 0
CELLSIZE 1000
NODATA_VALUE -999

Your ascii open in a text editor should look like this:

NCOLS 700
NROWS 1300
XLLCORNER 0
YLLCORNER 0
CELLSIZE 1000
NODATA_VALUE -999
0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 
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In terms of format this looks correct, but I think it may be a significant mistake to conflate zero with a NoData value. A zero is needed to indicate a zero wind speed, whereas a NoData value would indicate the absence of information about wind speed. –  whuber Jan 9 '13 at 19:44
    
I was assuming the NoData was 0, since @tedwalsh didn't gave this info. –  A.R Jan 9 '13 at 21:47
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I know these data very well. They are the slightly notorious NOABL windspeed data. You are also on the right track in converting them to an ASCII raster though. The header I created for them (many years ago) was as follows:

ncols 700
nrows 1300
xllcorner 0
yllcorner 0
cellsize 1000
nodata_value -999
0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 

It looks like you have used pipe symbols ('|') instead of 'l' for xll and yll in the header (which stand for 'lower left'). Also lose the numbers in brackets at the start of each row and the inverted commas too. Just make it space delimited. The numbers in brackets (as I'm sure you've worked out) denote the 100 kilometer start coordinates of the row of data (which is at a resolution of 1km - so there are a hundred values after each set of brackets). These numbers are not only redundant in an ESRI ASCII raster but will also 'break' it.

CAVEAT:
Use these data with caution. They are very old and were interpolated from a tiny handful of meteorological stations spread widely across the UK. They were also interpolated without taking any account of surface roughness and most other factors that influence windspeed at a local level. They are OK as a rough guide but are notorious for over estimating windspeed in many locations. For instance, I regularly find the NOABL 45m windspeed to often be comparable to wind speeds measured by anemometer at heights of 80m, but then again, since NOABL did not use any data from the Northern Isles, I have found NOABL to woefully underestimate windspeeds in Shetland. The furthest north of any of the few met stations (only 10 I think!) in the NOABL data was Wick.

All the warnings aside however, I have found that when used wisely, understanding the overestimation (and with a sprinkle of experience) I have found results of calculations of wind-farm yields using these data to be comparable to calculations using more up to date and rigorously collected data. However, don't take that as gospel because every site is different and actual wind speed is greatly effected by roughness, open reach, topography to name a few factors.

If windspeed is critical to your project, by all means use this as a starting point but don't bet the farm on it!

EDIT
To remove the numbers in brackets you could write a Python script to parse the data or an easy alternative is to:

  • open the file in a text editor, delete the first line ("Conversion of speed..."), globally replace all spaces with nothing and re-save with the extension '.csv'
  • Next open in a spreadsheet package setting the delimiter to be a semicolon (in Excel and OpenOffice Calc you will be asked when you open the file). Now select the first column which contains all the bracketed numbers, and delete it.
  • Insert six rows at the very start and add your header ("ncols 700" etc).
  • Finally save with the extension '.asc' but make sure your delimiter is a space on saving.
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Sylvester, Many thanks. Having a problem removing the numbers in brackets. Should the first few lines appear as below? nrows 1300 xllcorner 0 yllcorner 0 cellsize 1000 nodata_value -999 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 –  tedwalsh Jan 10 '13 at 9:08
    
@tedwalsh: that's pretty much it. I edited my answer above to add most of the first line to the 'code' sample. I have also added instructions on removing the brackets the easy way. If you are still struggling then send me a PM with your email address (find the Contacts section of my website) and I'll sort something out for you. –  MappaGnosis Jan 10 '13 at 9:44
1  
The NOABL data set was pretty neat for its time, but that time was almost 20 years ago. I remember working on/with it back then, too. Best not used for real work at typical wind turbine heights now. –  scruss Aug 12 '13 at 11:50
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