02-spatial-relationships-operations.ipynb

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   "source": [
    "# Spatial relationships and operations"
   ]
  },
  {
   "cell_type": "code",
   "execution_count": null,
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   "outputs": [],
   "source": [
    "%matplotlib inline\n",
    "\n",
    "import pandas as pd\n",
    "import geopandas\n",
    "\n",
    "pd.options.display.max_rows = 10"
   ]
  },
  {
   "cell_type": "code",
   "execution_count": null,
   "metadata": {},
   "outputs": [],
   "source": [
    "countries = geopandas.read_file(\"zip://./data/ne_110m_admin_0_countries.zip\")\n",
    "cities = geopandas.read_file(\"zip://./data/ne_110m_populated_places.zip\")\n",
    "rivers = geopandas.read_file(\"zip://./data/ne_50m_rivers_lake_centerlines.zip\")"
   ]
  },
  {
   "cell_type": "markdown",
   "metadata": {},
   "source": [
    "## Spatial relationships\n",
    "\n",
    "An important aspect of geospatial data is that we can look at *spatial relationships*: how two spatial objects relate to each other (whether they overlap, intersect, contain, .. one another).\n",
    "\n",
    "The topological, set-theoretic relationships in GIS are typically based on the DE-9IM model. See https://en.wikipedia.org/wiki/Spatial_relation for more information.\n",
    "\n",
    "![](img/TopologicSpatialRelations2.png)\n",
    "(Image by [Krauss, CC BY-SA 3.0](https://en.wikipedia.org/wiki/Spatial_relation#/media/File:TopologicSpatialRelarions2.png))"
   ]
  },
  {
   "cell_type": "markdown",
   "metadata": {},
   "source": [
    "### Relationships between individual objects"
   ]
  },
  {
   "cell_type": "markdown",
   "metadata": {},
   "source": [
    "Let's first create some small toy spatial objects:\n",
    "\n",
    "A polygon <small>(note: we use `.squeeze()` here to to extract the scalar geometry object from the GeoSeries of length 1)</small>:"
   ]
  },
  {
   "cell_type": "code",
   "execution_count": null,
   "metadata": {},
   "outputs": [],
   "source": [
    "belgium = countries.loc[countries['name'] == 'Belgium', 'geometry'].squeeze()"
   ]
  },
  {
   "cell_type": "markdown",
   "metadata": {},
   "source": [
    "Two points:"
   ]
  },
  {
   "cell_type": "code",
   "execution_count": null,
   "metadata": {},
   "outputs": [],
   "source": [
    "paris = cities.loc[cities['name'] == 'Paris', 'geometry'].squeeze()\n",
    "brussels = cities.loc[cities['name'] == 'Brussels', 'geometry'].squeeze()"
   ]
  },
  {
   "cell_type": "markdown",
   "metadata": {},
   "source": [
    "And a linestring:"
   ]
  },
  {
   "cell_type": "code",
   "execution_count": null,
   "metadata": {},
   "outputs": [],
   "source": [
    "from shapely.geometry import LineString\n",
    "line = LineString([paris, brussels])"
   ]
  },
  {
   "cell_type": "markdown",
   "metadata": {},
   "source": [
    "Let's visualize those 4 geometry objects together (I only put them in a GeoSeries to easily display them together with the geopandas `.plot()` method):"
   ]
  },
  {
   "cell_type": "code",
   "execution_count": null,
   "metadata": {},
   "outputs": [],
   "source": [
    "geopandas.GeoSeries([belgium, paris, brussels, line]).plot(cmap='tab10')"
   ]
  },
  {
   "cell_type": "markdown",
   "metadata": {},
   "source": [
    "You can recognize the abstract shape of Belgium.\n",
    "\n",
    "Brussels, the capital of Belgium, is thus located within Belgium. This is a spatial relationship, and we can test this using the individual shapely geometry objects as follow:"
   ]
  },
  {
   "cell_type": "code",
   "execution_count": null,
   "metadata": {},
   "outputs": [],
   "source": [
    "brussels.within(belgium)"
   ]
  },
  {
   "cell_type": "markdown",
   "metadata": {},
   "source": [
    "And using the reverse, Belgium contains Brussels:"
   ]
  },
  {
   "cell_type": "code",
   "execution_count": null,
   "metadata": {},
   "outputs": [],
   "source": [
    "belgium.contains(brussels)"
   ]
  },
  {
   "cell_type": "markdown",
   "metadata": {},
   "source": [
    "On the other hand, Paris is not located in Belgium:"
   ]
  },
  {
   "cell_type": "code",
   "execution_count": null,
   "metadata": {},
   "outputs": [],
   "source": [
    "belgium.contains(paris)"
   ]
  },
  {
   "cell_type": "code",
   "execution_count": null,
   "metadata": {},
   "outputs": [],
   "source": [
    "paris.within(belgium)"
   ]
  },
  {
   "cell_type": "markdown",
   "metadata": {},
   "source": [
    "The straight line we draw from Paris to Brussels is not fully located within Belgium, but it does intersect with it:"
   ]
  },
  {
   "cell_type": "code",
   "execution_count": null,
   "metadata": {},
   "outputs": [],
   "source": [
    "belgium.contains(line)"
   ]
  },
  {
   "cell_type": "code",
   "execution_count": null,
   "metadata": {},
   "outputs": [],
   "source": [
    "line.intersects(belgium)"
   ]
  },
  {
   "cell_type": "markdown",
   "metadata": {},
   "source": [
    "### Spatial relationships with GeoDataFrames\n",
    "\n",
    "The same methods that are available on individual `shapely` geometries as we have seen above, are also available as methods on `GeoSeries` / `GeoDataFrame` objects.\n",
    "\n",
    "For example, if we call the `contains` method on the world dataset with the `paris` point, it will do this spatial check for each country in the `world` dataframe:"
   ]
  },
  {
   "cell_type": "code",
   "execution_count": null,
   "metadata": {},
   "outputs": [],
   "source": [
    "countries.contains(paris)"
   ]
  },
  {
   "cell_type": "markdown",
   "metadata": {},
   "source": [
    "Because the above gives us a boolean result, we can use that to filter the dataframe:"
   ]
  },
  {
   "cell_type": "code",
   "execution_count": null,
   "metadata": {},
   "outputs": [],
   "source": [
    "countries[countries.contains(paris)]"
   ]
  },
  {
   "cell_type": "markdown",
   "metadata": {},
   "source": [
    "And indeed, France is the only country in the world in which Paris is located."
   ]
  },
  {
   "cell_type": "markdown",
   "metadata": {},
   "source": [
    "Another example, extracting the linestring of the Amazon river in South America, we can query through which countries the river flows:"
   ]
  },
  {
   "cell_type": "code",
   "execution_count": null,
   "metadata": {},
   "outputs": [],
   "source": [
    "amazon = rivers[rivers['name'] == 'Amazonas'].geometry.squeeze()"
   ]
  },
  {
   "cell_type": "code",
   "execution_count": null,
   "metadata": {},
   "outputs": [],
   "source": [
    "countries[countries.crosses(amazon)]  # or .intersects"
   ]
  },
  {
   "cell_type": "markdown",
   "metadata": {},
   "source": [
    "<div class=\"alert alert-info\" style=\"font-size:120%\">\n",
    "<b>REFERENCE</b>: <br><br>\n",
    "\n",
    "Overview of the different functions to check spatial relationships (*spatial predicate functions*):\n",
    "\n",
    "<ul>\n",
    "  <li>`equals`</li>\n",
    "  <li>`contains`</li>\n",
    "  <li>`crosses`</li>\n",
    "  <li>`disjoint`</li>\n",
    "  <li>`intersects`</li>\n",
    "  <li>`overlaps`</li>\n",
    "  <li>`touches`</li>\n",
    "  <li>`within`</li>\n",
    "  <li>`covers`</li>\n",
    "</ul>\n",
    "\n",
    "<p>\n",
    "See https://shapely.readthedocs.io/en/stable/manual.html#predicates-and-relationships for an overview of those methods.\n",
    "<p></p>\n",
    "See https://en.wikipedia.org/wiki/DE-9IM for all details on the semantics of those operations.\n",
    "</p>\n",
    "</div>"
   ]
  },
  {
   "cell_type": "markdown",
   "metadata": {},
   "source": [
    "## Spatial operations\n",
    "\n",
    "Next to the spatial predicates that return boolean values, Shapely and GeoPandas aslo provide analysis methods that return new geometric objects.\n",
    "\n",
    "See https://shapely.readthedocs.io/en/stable/manual.html#spatial-analysis-methods for more details."
   ]
  },
  {
   "cell_type": "markdown",
   "metadata": {},
   "source": [
    "For example, using the toy data from above, let's construct a buffer around Brussels (which returns a Polygon):"
   ]
  },
  {
   "cell_type": "code",
   "execution_count": null,
   "metadata": {},
   "outputs": [],
   "source": [
    "geopandas.GeoSeries([belgium, brussels.buffer(1)]).plot(alpha=0.5, cmap='tab10')"
   ]
  },
  {
   "cell_type": "markdown",
   "metadata": {},
   "source": [
    "and now take the intersection, union or difference of those two polygons:"
   ]
  },
  {
   "cell_type": "code",
   "execution_count": null,
   "metadata": {},
   "outputs": [],
   "source": [
    "brussels.buffer(1).intersection(belgium)"
   ]
  },
  {
   "cell_type": "code",
   "execution_count": null,
   "metadata": {},
   "outputs": [],
   "source": [
    "brussels.buffer(1).union(belgium)"
   ]
  },
  {
   "cell_type": "code",
   "execution_count": null,
   "metadata": {},
   "outputs": [],
   "source": [
    "brussels.buffer(1).difference(belgium)"
   ]
  },
  {
   "cell_type": "markdown",
   "metadata": {},
   "source": [
    "Another useful method is the `unary_union` attribute, which converts the set of geometry objects in a GeoDataFrame into a single geometry object by taking the union of all those geometries.\n",
    "\n",
    "For example, we can construct a single object for the Africa continent:"
   ]
  },
  {
   "cell_type": "code",
   "execution_count": null,
   "metadata": {},
   "outputs": [],
   "source": [
    "africa_countries = countries[countries['continent'] == 'Africa']"
   ]
  },
  {
   "cell_type": "code",
   "execution_count": null,
   "metadata": {},
   "outputs": [],
   "source": [
    "africa = africa_countries.unary_union"
   ]
  },
  {
   "cell_type": "code",
   "execution_count": null,
   "metadata": {},
   "outputs": [],
   "source": [
    "africa"
   ]
  },
  {
   "cell_type": "code",
   "execution_count": null,
   "metadata": {
    "scrolled": false
   },
   "outputs": [],
   "source": [
    "print(str(africa)[:1000])"
   ]
  },
  {
   "cell_type": "markdown",
   "metadata": {},
   "source": [
    "<div class=\"alert alert-info\" style=\"font-size:120%\">\n",
    "<b>REMEMBER</b>: <br><br>\n",
    "\n",
    "GeoPandas (and Shapely for the individual objects) provides a whole lot of basic methods to analyse the geospatial data (distance, length, centroid, boundary, convex_hull, simplify, transform, ....), much more than the few that we can touch in this tutorial.\n",
    "\n",
    "\n",
    "<ul>\n",
    "  <li>An overview of all methods provided by GeoPandas can be found here: http://geopandas.readthedocs.io/en/latest/reference.html</li>\n",
    "</ul>\n",
    "\n",
    "</div>\n",
    "\n"
   ]
  }
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