"Could not load source layer for INPUT" error when reprojecting flt-file with PyQGIS

Question

The data I have are ESRI Floating Point Data (so they are a raster type). The path I provided is a fake path, so that I don't show my real system paths.

I know that my code works with vector data, as I wrote this script, just for other data-types (.shp, .gpkg). Therefore, I assume, that the algorithm doesn't work with the flt-type.

... and indeed it doesn't. For reprojecting raster data, there is the algorithm warp.

So what I did yesterday was to use batch processing for applying the warp-function to the flt-data. The decision to not adapt the python script to warp the data but to use the batch-processing-functionality was just that it was a good opportunity to try it out (never worked with batches before).

---

I have a couple of .flt-files and I would like to reproject them to another CRS. In QGIS desktop, you can load the .flt-files by just dragging and dropping the file.

However, when I try to load the files in PyQGIS it says:

Could not load source layer for INPUT: path/to/3257559305385.flt not found

My code is just a function which uses the processing module. Is it somehow possible to load .flt-data in PyQGIS?

def reprojectFLTfiles(inputfolderpath, outputfolderpath, crs):
    for fltfile_name in os.listdir(inputfolderpath):
        if '.flt' in fltfile_name:
            if 'xml' not in fltfile_name:
                print(fltfile_name)
                INPUT = inputfolderpath + '/' + fltfile_name
                OUTPUT = outputfolderpath + '/' + fltfile_name
                print(INPUT)
                print(OUTPUT)
                processing.run("qgis:reprojectlayer",
                               {'INPUT':INPUT,
                                'TARGET_CRS':crs,
                                'OUTPUT':OUTPUT
                               }
                )

Answer 1

I would agree with @BenW's comment, that your binary floating-point files can not be found under this path inputfolderpath + '/' + fltfile_name. However, I could also say, that your input does not match any of the INPUT accepted data types. See, that "Reproject layer" geoalgorithm reprojects a vector layer in a different CRS. See the GDAL Vector drivers for more details.

⚠️ For raster data types, use the "Warp (reproject)", see the GDAL Raster drivers for more details.

Your binary floating-point file with a .flt extension should come with an ASCII header file that has a .hdr extension, see EHdr -- ESRI .hdr Labelled. You only specify the .flt file; however, there needs to be an existing .hdr file in the same directory with the same file name.

---

There are several suggestions to your code:

• check firstly alg input and output by means of the processing.algorithmHelp("qgis:reprojectlayer")

  INPUT: Input layer
  
      Parameter type:  QgsProcessingParameterFeatureSource
  
      Accepted data types:
          - str: layer ID
          - str: layer name
          - str: layer source
          - QgsProcessingFeatureSourceDefinition
          - QgsProperty
          - QgsVectorLayer
  
  OUTPUT: Reprojected
  
      Parameter type:  QgsProcessingParameterFeatureSink
  
      Accepted data types:
          - str: destination vector file, e.g. 'd:/test.shp'
          - str: 'memory:' to store result in temporary memory layer
          - str: using vector provider ID prefix and destination URI, e.g. 'postgres:…' to store result in PostGIS table
          - QgsProcessingOutputLayerDefinition
          - QgsProperty
  

• apply the isfile() function from the os.path library to check if a file exists

  if isfile(INPUT):
     ...
  

  Or, as was mentioned by @BenW, apply the exists() function. This function isdir() can also be handy for dirs.

• better to work with a normalized path, can be achieved with the normpath() function

  for fltfile_name in listdir(normpath(inputfolderpath)):
  

• two if-statements can be combined into one

  if '.flt' in fltfile_name and not ('xml' in fltfile_name):
     ...   
  

• make use of the join() function to get a proper file path

  INPUT = join(inputfolderpath, fltfile_name)
  

• use the Python Try Except blocks to handle errors and exceptions e.g. QgsProcessingException.

           try:
               params = {
                   'INPUT': INPUT,
                   'TARGET_CRS': crs,
                   'OUTPUT': OUTPUT
               }
               processing.run("qgis:reprojectlayer", params)
               return True
           except QgsProcessingException:
               return False
  

  The QGIS processing algorithm also has a checkParameterValues()-method:

  Checks the supplied parameter values to verify that they satisfy the requirements of this algorithm in the supplied context.

  The message parameter will be filled with explanatory text if validation fails. Overridden implementations should also check this base class implementation.

• it is not clear yet in which form you provide the crs value, but there is a class QgsCoordinateReferenceSystem(), that facilitates creation and work with CRS

  crs = QgsCoordinateReferenceSystem.fromEpsgId(4326)
  

• imports can be useful to make your code reusable in the future in standalone applications

  # imports
  import processing
  from qgis.core import QgsCoordinateReferenceSystem
  from os import listdir
  from os.path import isfile, join, normpath
  

• add some reprojectFLTfiles descriptions, see the PEP 20 – The Zen of Python for more inspiration

  def reprojectFLTfiles(inputfolderpath: str, outputfolderpath: str, crs: int) -> bool:
      """
      Reprojects a FTL file using a certain CRS
      Parameters:
      ==========
      :param inputfolderpath:
      :param outputfolderpath:
      :param crs:
      Returns:
      ==========
      :return: 
      """
  

So, in the end, it may look like this:

# imports
import processing
from qgis.core import QgsCoordinateReferenceSystem
from os import listdir
from os.path import isfile, join, normpath


def reprojectFLTfiles(inputfolderpath: str, outputfolderpath: str, epsg_code: int) -> bool:
    """
    Reprojects FTL file to a certain CRS
    Parameters:
    ==========
    :param inputfolderpath:
    :param outputfolderpath:
    :param crs:
    Returns:
    ==========
    :return:
    """

    crs = QgsCoordinateReferenceSystem.fromEpsgId(epsg_code)

    for fltfile_name in listdir(inputfolderpath):
        if '.gpkg' in fltfile_name and not ('xml' in fltfile_name):
            INPUT = join(inputfolderpath, fltfile_name)
            if isfile(INPUT):
                OUTPUT = join(outputfolderpath, fltfile_name)
                try:
                    params = {
                        'INPUT': INPUT,
                        'TARGET_CRS': crs,
                        'OUTPUT': OUTPUT
                    }
                    processing.run("qgis:reprojectlayer", params)
                    return True
                except QgsProcessingException as e:
                    print(f'Could not reproject this {INPUT} file. Ended with the following error:')
                    print(e)
                    return False


reprojectFLTfiles('D:/qgis_test/', 'D:/qgis_test/temp/', 4326)

With the code above, I could reproject my GeoPackage from EPSG:32637 into EPSG:4326.

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References:

• Error: Could not load source layer for INPUT
• Why am I getting 'Could not load source layer for INPUT: invalid value' for native:extractbyexpression algorithm on QGIS?
• Creating flt (floating point raster) with QGIS
• QgsProcessingException in Processing script QGIS3