Seems quite right.

Poison (or any other Json encoder for that matter) doesn’t support tuples, because they don’t exists in Json.

One way to solve the problem is to convert the tuple to a list

{48.51572187715065, 48.51572187715065} |> Tuple.to_list() |> Poison.encode
{:ok, "[48.51572187715065,48.51572187715065]"}

In your specific case, Geo supports encoding Points to a Json compatible format.
You have to use Geo.JSON.encode(point)

From their Github page

#Examples using Poison as the JSON parser

iex(1)> Geo.JSON.encode(point)
{:ok, %{ "type" => "Point", "coordinates" => [100.0, 0.0] }}

iex(2)> point = Poison.decode!("{ \"type\": \"Point\", \"coordinates\": [100.0, 0.0] }") |> Geo.JSON.decode
%Geo.Point{ coordinates: {100.0, 0.0}, srid: nil }

iex(3)> Geo.JSON.encode!(point) |> Poison.encode!
"{\"type\":\"Point\",\"coordinates\":[100.0,0.0]}"