How the lines are constructed
Astrocartography redraws a birth chart as a map of the world. The birth moment stays fixed while the observing location changes, and every place from which Neptune would have appeared exactly on the horizon or exactly on the meridian is joined into a line.
Jim Lewis assembled the technique in the 1970s. It is modern, and it needed computers to be practical. The older principle it borrows is angularity, the rule that a planet on one of the chart's angles is strengthened. Neptune was discovered in 1846, so its interpretive material is recent by any measure.
The four angles, and the shape of each line
The Neptune MC line runs through the longitudes where Neptune was culminating on the upper meridian. The IC line marks where it was at the lowest point, on the meridian beneath. Meridians are fixed circles running pole to pole, so both draw as straight vertical lines.
The AC line, where Neptune was rising in the east, and the DC line, where it was setting in the west, are curves. The horizon plane meets the ecliptic at an angle that changes with latitude, so the rising point of any body moves as you travel north or south. Near the poles the curves bend so hard they can look like a rendering fault. Most explanations of astrocartography present all four lines as though they were the same kind of object, which leaves readers unable to interpret what they are actually looking at.
What the tradition reads into Neptune's lines
Neptune signifies dissolution of boundaries, imagination, water, image making, sacrifice and things that are not what they appear to be. The MC line is read as public life bound up with image, film, music, care work or spiritual matters, and as a reputation that people project onto rather than see clearly. The named difficulty is being misread, and a career whose outline never quite firms up.
The IC line is read as a private life that is porous, restful for some accounts and directionless for others. The AC line is described as a place where a person feels indistinct, impressionable, and easily influenced, with escapism as the standard caution. The DC line reads through others, associated with idealised partners and with disappointment when the idealisation fails.
The vagueness of that vocabulary is the problem. A reading elastic enough to match any experience is not really being tested by anything, and it is fair to hold Neptune's lines to a stricter standard than the writing usually does.
Orb, and where the exactness stops
The usual orb is a few hundred kilometres. Seven hundred to eight hundred is the figure quoted in most sources, with the effect described as strongest exactly on the line. That is a convention, arrived at by repetition. Nothing measured it, and different practitioners set it differently without any way of adjudicating between them.
The honest framing applies with particular force here. Astrocartography is geometry applied to a symbolic system. The geometry is exact and computable, which is why the lines can be drawn precisely and why any competent program reproduces them. What a Neptune line means where it crosses a city is convention, and unusually loose convention at that. It is not a basis on which to move.
Parans, latitudes at which two planets are angular together, and relocated charts, which rearrange the houses for a new city, are the two additions most often used alongside the lines.
Questions people ask about this
What is a Neptune line read for?
Imagination, image, water, care and the softening of boundaries, with the angle setting the emphasis. The MC line points toward public image and the AC line toward personal impressionability.
Why are Neptune readings so vague?
Because the planet's signification is itself about indistinctness, the vocabulary tends to be elastic. A description broad enough to fit any experience is not saying much, and that is worth noticing.
Are all four Neptune lines drawn the same way?
No. The MC and IC lines lie on meridians and run straight north to south. The AC and DC lines follow the horizon and curve, because the horizon's tilt against the ecliptic varies with latitude.
How close to a Neptune line do you need to be?
The convention is about seven hundred to eight hundred kilometres, strongest on the line. That figure comes from custom in the literature rather than from measurement.
Is astrocartography ancient?
No. It was developed by Jim Lewis in the 1970s. Neptune was not discovered until 1846, so nothing about these lines has an ancient pedigree.
Should a Neptune line put me off a city?
No. The geometry is exact but the meaning is convention, and no line on a map is a reason to go somewhere or to stay away.