Distance mistake during grid creation

Question

I'm working on a script where I create a grid with the bounding box of a geodataframe, a width and a height (they might be equals for a square grid).

I don't know why but when I try to make a 10m square grid my script generate a 7m square grid.

All my process works with geodataframe in Pseudo Mercator (3857) who have meter for unit.

Here is the piece of script for the grid:

Xmin, Ymin, Xmax, Ymax = buffer_CH.total_bounds # coordonées de la bounding box du buffer de l'enveloppe concave

    cols = abs(int((Xmax - Xmin) / W))+1 # nombre de colonnes
    rows = abs(int((Ymin - Ymax) / H))+1 # nombre de lignes

    XleftOri, XrightOri, Ybottom, Ytop = Xmin, Xmin+W, Ymin, Ymin+H
    polygons = []
    print(XleftOri, XrightOri, Ybottom, Ytop)
    for r in range(rows):
        Xleft = XleftOri
        Xright = XrightOri
        for c in range(cols):
            polygon = Polygon([(Xleft,Ybottom),(Xright,Ybottom),(Xright,Ytop),(Xleft,Ytop)])
            polygons.append(polygon)
            Xleft = Xright
            Xright += W
        Ybottom = Ytop
        Ytop += H

    grid = gpd.GeoDataFrame({'geometry':polygons})
    grid.crs = {'init': 'epsg:3857'}
    grid.to_file("{0}/{1}_{2}_zonage-carre/{3}-grid.shp".format(os.chdir, date, folder_name, shp_name))

Issue seems to come from this operation:

rotate_grid = grid.rotate(angleinverse, origin=(0,0), use_radians=False) # Rotation
rotate_grid = gpd.GeoDataFrame(rotate_grid) # Conversion de la GeoSerie en GeoDataFrame
rotate_grid = rotate_grid.rename(columns={0:'geometry'}).set_geometry('geometry') # Identification de la colonne geometry
rotate_grid.crs = {'init': 'epsg:3857'} # attribution du crs
rotate_grid.to_file("{0}/{1}_{2}_zonage-carre/{3}-RotateGrid.shp".format(os.chdir, date, folder_name, shp_name))

Answer 1

Before saving to file assign the desired crs to your GeoDataFrame:

from fiona.crs import from_epsg
grid = gpd.GeoDataFrame({'geometry':polygons})
grid.crs = from_epsg(3857)

EDIT:

instead of using qgis measurement tool to compute the lenth of your polygons, use compute the length of the exterior Linear ring:

rotated_grid['perimetre'] = rotated_grid['geometry'].apply(lambda x: x.exterior.length)

so you can check if indeed it the polygons have changed size, or is it just an error due to the qgis measurement tool