pgRouting技术问题：如何用pgr_trsp从线串最近点发起路径规划？

Hey there! Let's work through this pgr_trsp issue you're hitting. It's frustrating when you've got the nearest point calculated and passed in, but the path still starts at the edge's source node instead of your desired starting spot. Here are some steps to debug and fix this:

1. Double-Check Your pgr_trsp Parameter Setup #

First, make sure you're using the function's offset parameters correctly. pgr_trsp lets you specify a starting position along an edge using the edge ID plus an offset distance—this is key to starting at your nearest point, not the edge's source node.

A correct call should look something like this:

SELECT * FROM pgr_trsp(
    'SELECT id, source, target, cost, reverse_cost FROM your_edges_table',
    -- Replace with your nearest point's edge ID and offset from the edge's source
    123, 45.6,
    -- Replace with your target node ID (or target edge + offset)
    789, 0.0,
    -- Set to true if you want to consider reverse edge costs
    true
);

The critical part here is the start_offset value (45.6 in the example)—this needs to be the exact distance from the edge's source node to your calculated nearest point along the edge.

2. Verify Your Offset Calculation #

It's easy to mess up the offset value. Use PostGIS functions to confirm you're calculating it correctly:

SELECT
    id,
    ST_Length(geom) * ST_LineLocatePoint(geom, ST_SetSRID(ST_MakePoint(<your_lon>, <your_lat>), 4326)) AS calculated_offset
FROM your_edges_table
WHERE id = <nearest_edge_id>;

This gives you the proportional distance from the edge's source node to your nearest point. If you're using a different SRID, adjust the ST_SetSRID value accordingly.

3. Check Edge Direction #

pgrouting edges are directional! If your nearest point is on the "reverse" side of the edge (closer to the target node than the source), passing a positive offset might not work as expected. Try one of these fixes:

• If your graph is undirected, set the last parameter of pgr_trsp to true to enable reverse cost consideration.
• Calculate the offset from the edge's target node instead, and use the reverse edge (swap source/target) in your pgr_trsp call.

4. Try Splitting the Edge (If All Else Fails) #

If adjusting offsets and direction doesn't solve it, you can temporarily split the edge to create a new node at your nearest point. Here's a quick outline:

1. Use ST_Split to split the original edge into two segments at your nearest point.
2. Create a new node entry for the split point in your nodes table.
3. Update your edges table to replace the original edge with two new edges (source → new node, new node → target).
4. Run pgr_trsp using the new node as your starting point.

This method guarantees your path starts exactly at the nearest point, though it requires modifying your graph temporarily.

内容的提问来源于stack exchange，提问作者Logan M