What transit lines are
Your natal lines are fixed: they mark where each planet was rising, setting, overhead or underfoot at the moment you were born, and they never move. Transit lines put the planets of another date on the same map. A transit MC line marks the places where a planet of that date stands on the Midheaven of your birth chart relocated there, and a transit ASC line the places where it rises in that relocated chart. Jim Lewis, the astrologer who developed Astro*Carto*Graphy in the 1970s, completed the method in 1982 as Cyclo*Carto*Graphy, which maps the transiting outer planets together with the progressed inner ones; others say cyclocartography, transit astrocartography or simply transit lines. The map here draws transits, not progressions.
How they are drawn
Every line on an astrocartography map comes from two things: where the planet is among the stars, and how the Earth was turned at your birth, its sidereal time. A transit line keeps the sidereal time of your birth and takes the planet's position on the chosen date. That is what makes transit lines personal. Two people who look at the same date see different transit lines, because the same planets are laid over different birth moments. On your astrocartography map, tick "Show transit lines" in the Lines tab and pick any date: the map draws the planets at noon UTC on that day as wide pale bands under your natal lines. The Moon is left out, since it crosses a whole sign in two and a half days.
How fast each line moves
A transit MC or IC line is a meridian, and it moves exactly as far east or west as the planet moves in right ascension. The Sun's line moves about a degree of longitude a day and goes round the world in a year; Mars takes about two years. Jupiter's line moves about 30° a year, some 3,400 km at the equator, while Pluto's moves under 2°. Every planet but the Sun also turns retrograde, and its line then moves west for a while, which is why the slow planets cross the same place two, three or four times. The table below gives each planet's figures, measured on this site's ephemeris.
Timing a place
Tap a city on the map, or search for one, and the place reading lists when the transit lines of Jupiter, Saturn, Uranus, Neptune and Pluto pass within 250 km of it over the next two years, with the dates and the closest approach. At 45° latitude a Jupiter line stays near a place for about a month per visit, and about half of Jupiter's visits bring two or three passes as it turns retrograde. Saturn stays about six months, usually in two passes; Uranus about 17 months; Neptune about 2.8 years; Pluto about 4.4 years, usually in four passes. The Sun's and the inner planets' lines pass in under ten days, too briefly to plan a move around, though some astrologers use them to time a trip.
Reading a transit line
The tradition reads a transit line with the themes of the natal line of the same planet and angle, but as a passing influence: a season rather than a setting. A transiting Jupiter line over a city is read as a favorable window there, a Saturn line as a time of work and responsibility, and the outer planets as long chapters of change. Astrologers give the most weight to transits that cross places already strong in the natal map, where they read the transit as switching on what the birth chart promised. The guide to reading your map covers the natal lines, and the transit chart shows the same planets against your birth chart without the map.
Transits, relocation and the natal map
Transit lines do not replace your natal lines. They add timing to them. A place with no natal lines nearby is read as quieter whatever passes over it, and a place on a strong natal line keeps that line whether or not a transit is crossing. Most astrologers who use transit lines look at them last: first the natal lines and parans, then the relocation chart for a city, and only then the transits, to choose when rather than where. Like every reading of astrocartography, it is a tradition, not a measurement.