Tiling and Transcribing a Larger Map

The first map I transcribed using Azure Cognitive Services in a previous post was fairly small in size. As mentioned in the post about tiling, Azure Cognitive Services has limits. I’m using the free tier which accepts files up to 4mb in size. A map I am working with is 90mb, so it needs to be divided into tiles smaller than 4mb, like below.

Map divided into tiles.

Some of the map tiles have no text, so there is no point transcribing them. I manually eliminated the empty tiles.

Only tiles with text on them are selected for transcription.

The first time I transcribed a map, I placed the image on a website so Azure Cognitive Services could reference it as a URL. With the tiles, it’s easier to process them as local files and pass them to Azure. The program opens each tile as an image and sends it to Azure Cognitive Services with the computervision_client.read_in_stream() method.

def recognize_text_in_local_image(local_image_handwritten_path):
    local_image_handwritten = open(local_image_handwritten_path, "rb")
    # Call API with image and raw response (allows you to get the operation location)
    recognize_handwriting_results = computervision_client.read_in_stream(local_image_handwritten, raw=True)

The text in the tiles is transcribed and can be placed geographically.

The trouble with tiles.

Tiling is process that cuts both ways, it divides the map into smaller pieces that can be processed but it also cuts up the text. Below is part of the map (row 5, columns 2 and 3.) The names Sol.n Stewart and Pet Bruner are split over two images

Sol.n Stewart is transcribed as 2 different names: “Soln” and “tewart”. Pet. Bruner loses the er of his last name:

Tiling splits names that span tiles.

Overlapping the tiles solves this so that each name is displayed fully on at least 1 tile. The Tile_tiff class has a means to overlap:

tt = Tile_tiff('/content/drive/MyDrive/MaskCNNhearths/HopewellFurnace/UnionTshp_BerckCtyAtlas_1860_georef.tif',
tt.tt_tile_pixel_width_overlap = 200
tt.tt_tile_pixel_height_overlap = 100


With the tiling, Sol.n Stewart regains a full name. Pet Bruner’s name is also made whole but gains the E. from E. Haas. However, where tiles overlap, such as on the right, there is duplication. A. Lord’s name at the bottom right is fully duplicated while T. Jones has a second version “T. Jon”.

Let’s get rid of these duplicates, but not lose useful names.

To find duplicates, the program compares all of the polygons containing text to others using: intersect_poly = poly1.intersection(poly2)

If the polygons don’t intersect, an empty polygon is returned and we know the 2 polygons are not duplicates.

If the polygons do intersect, it’s a question of how much they overlap. I assume that the smaller polygon is the one to discard if they do intersect. Some polygons will intersect that are not duplicates. The program checks to discard only smaller polygons covered 80% or more by a larger intersecting polygon. Below is the code to remove the duplicates followed by some edited output.

glist = line_text_df['geometry']
tlist = line_text_df['text']
rows_to_remove = []
for i in range(len(glist)):
    for j in range(i + 1, len(glist)):
        intersect_poly = glist[i].intersection(glist[j])

        if(not intersect_poly.is_empty):
            if(glist[i].area < glist[j].area):
                # glist[i] is smaller
                if(intersect_poly.area/glist[i].area >.8):
                    print("remove i : ", i, j, "{:3.2%}".format(intersect_poly.area/glist[i].area), int(intersect_poly.area), " Remove: ", tlist[i], int(glist[i].area), " Keep: ", tlist[j], int(glist[j].area))
                if(glist[i].area >= glist[j].area):  
                    if(intersect_poly.area/glist[j].area >.8):
                        print("remove j : ", i, j, "{:3.2%}".format(intersect_poly.area/glist[j].area), int(intersect_poly.area), " Keep: ", tlist[i], int(glist[i].area),  " Remove: ", tlist[j], int(glist[j].area))

Intersection coverage: 97.55% Keep: “Wag. stop” area: 131287. Remove: “Wag. stop” area: 130344.

Intersection coverage: 100.00% Remove: “T. Jon” area: 87115. Keep: “T. Jones” area: 128244.


Below is a close up of text polygons after duplicates are removed. There is only one T. Jones and Wag. Stop which is good. Although the Haas box at the bottom intersects with “Pet. Bruner E.”, it’s not removed as duplicate, which is also good. It’s too bad there is a transcription recognition error here due to word alignment and 2 nearby houses marked on the map.

Here is the resulting map:

Resulting map with duplicate labels removed.

The program to tile the maps is here. The program to transcribe the tiles and remove duplicates is here.

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