[gdal-dev] ReadAsArray

[Stefan Keller]

Hi Yuta and Even

Excuse me when I creep in that discussion - but I'm acutally having
quite a similar beginners question (I'm rather a vector guy :-) ):

Yuta: Can you post the code fragment here in order to make benchmarks?

I have a GeoTIFF (representing heights from SRTM3) in Mercator CRS of
the size of a country.
And I have a position (in lat/lon) and a radius (in m) as input.
Now, I'd like to read out all grid cells within a radius in order to
calculate the highest point nearby.
So I'd like to read out a subset of grid cells/pixels in the most efficient way.

Here's some code snippet:
  import gdal
  from gdalconst import *
  filename = 'my.tif'
  dataset = gdal.Open(filename, GA_ReadOnly)
  band = dataset.GetRasterBand(1)
  scanline = band.ReadRaster( 0, 0, band.XSize, 1,band.XSize, 1, band.DataType)
  ...

Following questions arise:
1. Are there alternatives reading out the sub-matrix
2. How to calculate the grid cell position in Mercator and lat/lon?

Yours, Stefan


2015-04-03 13:16 GMT+02:00 Yuta Sato <yutaxsato at gmail.com>:
> Thank you very much "Even Rouault" for making me understood.
>
> On Fri, Apr 3, 2015 at 8:13 PM, Even Rouault <even.rouault at spatialys.com>
> wrote:
>>
>> Le vendredi 03 avril 2015 12:46:48, Yuta Sato a écrit :
>> > Dear Even Rouault:
>> >
>> > Thank you very much.
>> > What about setting these parameters "used with .ReadAsArray()", though I
>> > did not know their meanings?
>> > buf_xsize=None, buf_ysize=None, buf_obj=None
>>
>> (Please keep the list CC'ed)
>>
>> buf_xsize and buf_ysize are the equivalents of nBufXSize and nBufYSize in
>> GDALRasterBand::RasterIO()
>> http://gdal.org/classGDALRasterBand.html#a75d4af97b3436a4e79d9759eedf89af4
>> i.e. to do downsampling or upsampling of original data.
>>
>> buf_obj can be used to "recycle" an existing numpy array of the
>> appropriate
>> size instead of allocating a new one.
>>
>> >
>> >
>> > On Fri, Apr 3, 2015 at 7:43 PM, Even Rouault
>> > <even.rouault at spatialys.com>
>> >
>> > wrote:
>> > > Le vendredi 03 avril 2015 12:22:00, Yuta Sato a écrit :
>> > > > Dear Respected GDAL Developers and Users:
>> > > >
>> > > > What parameters should I set beforehand in order to accelerate the
>> > >
>> > > reading
>> > >
>> > > > of a GeoTiff file?
>> > > >
>> > > > I am using as follows:
>> > > >
>> > > > data =
>> > > > src_dataset.GetRasterBand(1).ReadAsArray(xoff,yoff,xsize,ysize)
>> > > >
>> > > > Does setting the following parameters accelerate?
>> > > >
>> > > > GDAL_CACHEMAX, GDAL_SWATH_SIZE
>> > > >
>> > > > I'm using gdal python.
>> > >
>> > > Yuta,
>> > >
>> > > Increasing GDAL_CACHEMAX might accelerate in case of repeated reads on
>> > > windows
>> > > that are identical or overlapping already read windows. Or if the way
>> > > you
>> > > read
>> > > the raster doesn't follow its block shape : for example if the raster
>> > > is
>> > > organized by lines/strips and you read by square blocks, or the
>> > > reverse
>> > > situation.
>> > >
>> > > GDAL_SWATH_SIZE is only used by CreateCopy().
>> > >
>> > > Even
>> > >
>> > > --
>> > > Spatialys - Geospatial professional services
>> > > http://www.spatialys.com
>>
>> --
>> Spatialys - Geospatial professional services
>> http://www.spatialys.com
>
>
>
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