Converting a quantized topojson string to geojson fails to dequantize the coordinates

[mattijn]

Converting a quantized topojson string to geojson fails to dequantize the coordinates

[mattijn]

Thanks for your words. You are right that it is currently geared towards geographical data to topojson. Better support for the other direction would be much welcome indeed.

There is already some within the code that makes this possible, but this could probably much improved. PR's definitely welcome!

Two possibilities:
First, if you parse a string into Topology(), where the string contains "type": "Topology" it is tried to serialise as geojson, and then convert to topojson again:

https://github.com/mattijn/topojson/blob/master/topojson/core/extract.py#L585

I can imagine that that is cumbersome. Based on some tests I also get strange outputs.

Second, there is another route. Parse your topojson as a dict and then use the serialise_as_geojson function:

import topojson as tp
from topojson.utils import serialize_as_geojson
import geopandas as gpd

# create a topojson dictionary
data = tp.utils.example_data_africa()
topo = tp.Topology(data)
topojson_dict = topo.to_dict()

?topojson_dict

Type:        dict
String form: {'type': 'Topology', 'objects': {'data': {'geometries': [{'id': '1', 'type': 'Polygon', 'properti <...> [750884, 541353], [-8944, -6372], [-10232, 32], [-11708, -3243], [-9248, 3099], [-5987, -3779]]]}
Length:      5
Docstring:  
dict() -> new empty dictionary
dict(mapping) -> new dictionary initialized from a mapping object's
    (key, value) pairs
dict(iterable) -> new dictionary initialized as if via:
    d = {}
    for k, v in iterable:
        d[k] = v
dict(**kwargs) -> new dictionary initialized with the name=value pairs
    in the keyword argument list.  For example:  dict(one=1, two=2)

# objectname is specific in each topojson file
geojson_dict = serialize_as_geojson(topojson_dict, objectname='data') 

# check if it is correct geojson
gdf = gpd.GeoDataFrame.from_features(geojson_dict["features"])
gdf.plot()

Btw, the dequantization on arcs happens here:

https://github.com/mattijn/topojson/blob/master/topojson/utils.py#L445

The reason why there is a dequantization of multi(points) in the geometry function is because there is only a top-level storage of arcs in the topojson specification and strangely not for the (multi)point.