I'm looking to compare changes to OSM over time for a given location (e.g., NYC). I've created OSM graphs of NYC at two different timestamps using the overpass_settings in ox.config(). I'd like to find all edge variations between the two OSM graphs (e.g., edges moving or being added or removed due to construction). I started with using a concatenation of u, v, key, and geometry as a unique id for each edge in both of the graphs (the idea of including geometry as part of the unique id was to capture the variation between the "same" edge at two different timestamps). However, from what I've read and also seen from manually exploring the two graphs as shapefiles, u, v, and key are not immutable over time. U, v, and key create a stable unique id within any particular OSM graph, but they may not create a stable unique id over time.
Is this an inherent limitation to OSM or is there a way to compare edge changes over time? If the above isn't clear, this conversation from the SharedStreets project describes what I'm looking to do, albeit within the SharedStreets framework.
Related
I have a PostGreSQL table that has a geometry type column, in which different simple polygons (possibly intersecting) are stored. The polygons are are all areas within a city. I receive an input of a point (latitude-longitude pair) and need to find the list of polygons that contain the given point. What I have currently:
Unclustered GiST index defined on the polygon column.
Use ST_Contains(#param_Point, table.Polygon) on the whole table.
It is quite slow, so I am looking for a more performant in-memory alternative. I have the following ideas:
Maintain dictionary of polygons in Redis, keyed by their geohash. Polygons with same geohash would be saved as a list. When I receive the point, calculate its geohash and trim to a desired level. Then search in the Redis map and keep trimming the point's geohash until I find the first result (or enough results).
Have a trie of geohashes loaded from the database. Update the trie periodically or by receiving update events. Calculate the point's geohash, search in the trie until I find enough results. I prefer this because the map may have long lists for a geohash, given the nature of the polygons.
Any other approaches?
I have read about libraries like GeoTrie and Polygon Geohasher but can't seem to integrate them with the database and the above ideas.
Any cues or starting points, please?
Have you tried using ST_Within? Not sure if it meets your criteria but I believe it is meant to be faster than st_contains
I have the following graph with 2 different parameters called p and t.
Their relationship is experimentally found. Manually by knowing (t,p), you can simply find the area number (group) of the point based on where it is located. For example, point M(t,p), locates in area 3 and belongs to group number 3. However, I would like to write a code/logical approach which automatically finds the group numbers. therefore when it reads (t,p) it will find the location of the point and give the group/Area number it belongs.
Is there any solution in Matlab for this scope? Graph
If you have the Image Processing Toolbox and your contours are closed, you can use imfill to fill them up (a bit like the bucket tool in Paint) and assign different values to each filled up region. Does this make sense to you? Let me know if you would like more detail.
Marta
So for the first time I'm gonna do a project that involves maps and layers on top of maps which have many points and many polygons on them.
I have the tendency to create separate tables for points and polygons and then create many-to-many relationships between them and the layers table. If I do that I end up with 5 tables: points, polygons, layers, layers_points and layers_polygons.
However, I see PostGIS also offers types called MULTIPOINT and MULTIPOLYGON. If I use those types then I could put it all in the layers table. I guess that would make queries faster, because I need less joins. However, I'm not sure if later I might regret it, if it means that working with the individual points and polygons becomes impossible. I'm not even sure yet if it will be necessary to work perform calculations on the individual points and polygons, but it would be nice to know whether that's possible or not in both approaches.
So basically I'm asking, what the pros and cons are of these different approaches?
In general, you would consider using multipolygons to represent entities that have disjoint surfaces (for example, the geometry of Alaska) or other topologies that you can't represent as polygons. The key here is that a single entity needs to be expressed with a multipolygon
What you wouldn't do is group unrelated polygons into a multipolygon, because you won't be able to perform queries at a child polygon level, unless you extract the rings into another geometry. If the polygons are unrelated, chances are you will need to query them individually. Even if they share a layer, you can manage that relation with business logic without merging them as they aren't representing the same entity.
Keep in mind that geometry tools in the frontend won't necesarilly treat multipolygons as a valid geometry or a multi object. Algorithms of point-in-polygon that looks like your use case, won't necesarilly work when checking if a point is contained in a multipolygon.
Tools like Wicket.js (transform from/to WKT/geojson/native objects) don't support multipolygons. Google maps api v3 doesn't support multipolygons except for the data layer (but you can't operate on the data layer as you would on a polygon feature). Turf.js has operations that would run on a Featurecollection containing several polygons, yet not over a multipolygon.
Without knowing your exact use case, that's the best I can tell you, and TL/DR: keep your polygons as they are.
I have a OSM node Id/ latitude-longitude for a point in the road(say point Z). How do I find the previous x points that I need to travel to reach Z in all directions? I was thinking overpass API could help me. But it is able to return points only with tags. I am not able to get it return the node Ids on the road/way.
Can you please suggest any API/tutorial that could help?
if i'm not wrong what you are asking is: given a osm node id with coordinate x and y what are all points to do in order to arrive there from a starting point?
if this is the question well this is a graph oriented question; you should create a grah and then use some algorithm in order to find all the routes between starting point and end point; you should use some graph oriented software.. something like neo4j and spatial contrib (https://github.com/neo4j-contrib/spatial)
In past i built a project where i read an osmfile, create a graph and used A* algorithm; you may give to it a look https://github.com/angeloimm/neo4jAstarTest
I suggest to get started by reading about OSM elements, especially nodes and ways. Afterwards take a look at OSM XML format. It might also help to open an OSM editor (e.g. iD) and to take a look at the raw data.
Nodes don't have any order or "next node" themselves. Nodes can be part of one or multiple ways. Each way references a list of ordered nodes. So you have to look at all ways a node belongs to, then look at the way's node list to determine the previous and next nodes. If the node is at the start or end of a way then you have to look if there are one or more consecutive ways. Consecutive ways share the same node at their start/end.
I imported the OSM data for Switzerland in Postgres and I am interested in getting the road data of a continuous part of a highway (I know the name),that is, the part that connects two specific cities. The highway is quite big (A1) and connects a lot of cities together.
I am not sure how the sequence of road segments is stored in postgres (ie, how one knows that one road segment is directly after the other). How should I query Postgres to get a linestring with the route from on city to another? I can visualize the data of the whole highway (which spans multiple cities) in QuantumGis by doing the query:
select osm_id,way from planet_osm_roads where highway='motorway' and ref='A1';
but I don't know how to only get the osm_ids that I am interested in, in the order they appear in the route. I do not want to do a bounding box constraint in the where clauses because I am looking for a general solution and also, I am still not sure how the order of the sequence of road segments is saved.
The way I did that was to use pgrouting, namely, their pgr_dijkstra algorithm. I loaded the OSM data into a format fit for use by pgrouting using the osm2pgrouting tool.