How does a geocoder "know" which end of a line is the "to" and "from" ends?

[Jackson_CountyMissouri]

This is sort-of a follow-up to my question yesterday.

Let's say I have a street line. I drew it from left to right. That is, if it has 2 nodes, node #1 will be on the left and node #2 will be on the right.

I then have FromLeft, FromRight, ToLeft and ToRight fields.

Even though I drew it from left to right, the flow of the addresses is right to left. So, for the fields I would put something like:

FromLeft: 1

FromRight: 2

ToLeft: 9

ToRight: 10

So the *actual* arrangement of the line would be:

10          2

-------------

9           1

Let's say a user uses my geocoder and enters an address of 3. 3 "should" be below the line a little to the left of the 1.

However, since I drew my line from left to right, will the geocoder "think" the 3 should be above the line a little bit to the right of the 10? After all, there's nothing that says the "right" or "left" of the line should be "above" it or "below" it in my scheme. It's just a line containing two nodes, and some numbers associated with it.

In other words, does the order of the nodes determine the direction of the flow of addresses (and thus, the right/left sides of the line), or does it figure that out some other way? Will I always have to draw my line in the direction of the flow of addresses (or alternatively, make the "to" and "from" and "right" and "left" numbers "backwards")?

Hope that made sense.

[RobertBorchert]

Line Direction

FromLeft: 1   The beginning number on the LEFT side of the line

FromRight: 2 The beginning number on the RIGHT side of the line.

ToLeft: 9

ToRight: 10

However, more complex geocoders can be devised based on the rules for a particular county. 

One of the great blunders by the government when they mandated E911 was they give the counties basic guidelines and let them choose the particulars themselves.

I started GIS in E911 Creation. Some counties use a 4 quadrant system, some use a corner of the county as 0,0. But it can be any corner.

Meaning County 1 has addressing that starts in the SW corner and increases North and East.  Then to make matters better they can then choose how many address there are per mile. And then decide on which side of the road will be even and which side odd.

Our company tried to standardize how the counties we developed did things.  We went SE corner with Odd on the left, evens on the right. 1000 address's per mile.

[Jackson_CountyMissouri]

"FromLeft: 1   The beginning number on the LEFT side of the line

FromRight: 2 The beginning number on the RIGHT side of the line."

Yeah but, how does the geocoder "know" which is the left and which is the right side of the line? If you start from one side, "left" will be above the line (in my drawing above). If you start from the other side of the line, "left" will be below the line. How does the geocoder "know" which side to start at?

[BruceHarold]

The line geometry determines left and right.

[Jackson_CountyMissouri]

All W-to-E streets have to have their first node on the left, and all E-to-W streets have to have their first node on the right?

[BruceHarold]

No, they can go either direction but whichever direction they go determines the left/right parity.

It is common practice to digitize your data in ascending address direction but in your boustrophedonic example from yesterday you can't so have to pay attention to from/to values on each side.

[RobertBorchert · Accepted solution]

In your map the line has a specific beginning and end.  If it is in the wrong direction you may need to flip the line.