Saturday, August 26, 2023

How to Calculate Scale Factor for Scaling Objects in AutoCAD

 If you are scaling an object in AutoCAD using the "SCALE" command, you will be prompted to enter scale factor. In as much as the scale factor could be entered on the fly, it would be proper to know the exact value that needed to be entered to scale an object in a precise manner.

To calculate the 'Scale Factor', lets consider the points P1 and P2 seen above. The distance expected is 407.97m, but when measured using the measuring tool the measured distance was 161.64m.

So, the 'Scale Factor' will be Expected distance ÷ Measured distance. That is: 407.97/161.64 = 2.523942093541203

Now select the whole drawing and perform the scaling using the 'Scale Factor' calculated above. The distances will be scaled properly and when measured using the measuring tool, it will return the correct expected distance.

Note: For stubborn objects such as 'Rotated Dimensions', the 'Scale Factor' above will not work as expected. A work around is to create a new dimension style and set the 'Scale Factor' to 1/'Scale Factor'. That is 1/2.523942093541203 in the case above.

That is it!

Tuesday, August 15, 2023

My Collection Inkscape Graphics

 All made with INKSCAPE not Adobe or any other commercial software. My inspirations came from the following YouTube channels on Inkscape tutorial: LogosByNick, SimpleShapes, IronEchoDesign, CreateForFree, SweaterCatDesigns and 2dgameartguru.

Sunday, August 13, 2023

How to resize multiple shapes in Inkscape

 I was preparing a client's map in Inkscape vector graphics software (that was the project requirement since the client doesn't work with GIS software). I converted the shapefile into SVG to allow me manipulate the map in Inkscape. You can do the conversion using the GIS tool or use online tools like Aspose or Mapshaper.

So, I stumbled on a huddle where I needed to resize over 2000 points (cycle objects) as seen above.

Now how to I individually select and resize over 2000 objects? Luckily inkscape has this Transform >> Scale tool that could be applied to each individual object.

You can access this tool using "Ctrl + Shift + M" or go to menu "Object >> Transform" as seen below.

On the Scale table set the size you want and check "Apply to each object separately".

That is it!

Saturday, August 5, 2023

PyQGIS - Export features based on selection from another layer

 We have states layer we want to use for selecting and exporting features from other layers that are within a selected state from the states' layer.

To achieve this task, we need to have basic understanding of the following:-

  1. Working with os and pyqgis module
  2. Read all layers listed on the layer's panel
  3. Understand how to get path to layer
  4. Make selection on layer
  5. Export selected features to new shapefile

Lets talk about each of the above separately.

Understanding how to work with OS and PyQGIS modules

One of the commonly used modules in python is the Operating System (OS) module. It provides functions for interacting with the operating system. Specifically in this task we will use the os module to get the base name of a shapefile from a given path.

Let say the shapefile path is this:


Then this will return its base name as 'NG_Admin.shp'

import os

Check the "dir(os)", you will see that we can do a lot more than the above including the following;-

'DirEntry', 'F_OK', 'GenericAlias', 'Mapping', 'MutableMapping', 'O_APPEND', 'O_BINARY', 'O_CREAT', 'O_EXCL', 'O_NOINHERIT', 'O_RANDOM', 'O_RDONLY', 'O_RDWR', 'O_SEQUENTIAL', 'O_SHORT_LIVED', 'O_TEMPORARY', 'O_TEXT', 'O_TRUNC', 'O_WRONLY', 'P_DETACH', 'P_NOWAIT', 'P_NOWAITO', 'P_OVERLAY', 'P_WAIT', 'PathLike', 'R_OK', 'SEEK_CUR', 'SEEK_END', 'SEEK_SET', 'TMP_MAX', 'W_OK', 'X_OK', '_AddedDllDirectory', '_Environ', '__all__', '__builtins__', '__cached__', '__doc__', '__file__', '__loader__', '__name__', '__package__', '__spec__', '_check_methods', '_execvpe', '_exists', '_exit', '_fspath', '_get_exports_list', '_walk', '_wrap_close', 'abc', 'abort', 'access', 'add_dll_directory', 'altsep', 'chdir', 'chmod', 'close', 'closerange', 'cpu_count', 'curdir', 'defpath', 'device_encoding', 'devnull', 'dup', 'dup2', 'environ', 'error', 'execl', 'execle', 'execlp', 'execlpe', 'execv', 'execve', 'execvp', 'execvpe', 'extsep', 'fdopen', 'fsdecode', 'fsencode', 'fspath', 'fstat', 'fsync', 'ftruncate', 'get_exec_path', 'get_handle_inheritable', 'get_inheritable', 'get_terminal_size', 'getcwd', 'getcwdb', 'getenv', 'getlogin', 'getpid', 'getppid', 'isatty', 'kill', 'linesep', 'link', 'listdir', 'lseek', 'lstat', 'makedirs', 'mkdir', 'name', 'open', 'pardir', 'path', 'pathsep', 'pipe', 'popen', 'putenv', 'read', 'readlink', 'remove', 'removedirs', 'rename', 'renames', 'replace', 'rmdir', 'scandir', 'sep', 'set_handle_inheritable', 'set_inheritable', 'spawnl', 'spawnle', 'spawnv', 'spawnve', 'st', 'startfile', 'stat', 'stat_result', 'statvfs_result', 'strerror', 'supports_bytes_environ', 'supports_dir_fd', 'supports_effective_ids', 'supports_fd', 'supports_follow_symlinks', 'symlink', 'sys', 'system', 'terminal_size', 'times', 'times_result', 'truncate', 'umask', 'uname_result', 'unlink', 'unsetenv', 'urandom', 'utime', 'waitpid', 'waitstatus_to_exitcode', 'walk', 'write'

On the other hand, PyQGIS is the QGIS Python API. It allows users to automate workflow and extend QGIS with the use of Python libraries. Specifically in this task we will use the following PyQGIS functions:-


Read all layer listed on the layer panel

To read all layers listed on the layer's panel, use this function QgsProject.instance().mapLayers()

The function returns a dictionary of the layers which you can iterate over to perform further processing.

Get layer path

The .source() method is used to retrieve path to a layer. We need this specifically to export the selection we will make below and also to create file name and path to save the selected features.

Making selection

There are several ways to make selections. However, here we will make use of selectByExpression method and run the native selectbylocation in the method.

Exporting selected features

The selected features will be exported using the QgsVectorFileWriter.writeAsVectorFormat method.

The final code

# Program to export features based on selected state(s)

import os

# Read all layers listed on the layers panel into a dict...
layer_dict = QgsProject.instance().mapLayers()
# Get all the layer names...
layer_names = [ for layer in layer_dict.values()]

for lyr_name in layer_names:
    # Layer to select from
    # Read Layer by name...
    layer1 = QgsProject.instance().mapLayersByName(lyr_name)[0]
    layer1_path = layer1.source()

    # Layer to base the selection (this is always the states layer)
    # Read Layer by name...
    layer2 = QgsProject.instance().mapLayersByName("NGA_adm1")[0]
    layer2_path = layer2.source()

    # Do selectByExpression on layer2...
    state = 'Benue'
    layer2.selectByExpression(f'"state_name" = \'{state}\'')

    # QGIS run processing script...."native:selectbylocation", 
        'INTERSECT':QgsProcessingFeatureSourceDefinition(layer2_path, selectedFeaturesOnly=True, featureLimit=-1, geometryCheck=QgsFeatureRequest.GeometryAbortOnInvalid),

    # Make file name...
    fn = state +'__'+ os.path.basename(layer1_path)

    # Save selected features to new shapefile...
    newfile_name = f'C:/Users/`HYJ7/Desktop/Working/Fiverr/2023/07-July/Clip All Layers/SHP/00/{fn}'
    writer = QgsVectorFileWriter.writeAsVectorFormat(layer1, newfile_name, 'utf-8', driverName='ESRI Shapefile', onlySelected=True)

    # To add the newly exported layer to map canvas...
    # iface.addVectorLayer(newfile_name, '', 'ogr')


That is it!