One more tool in the astrologer’s kit is the locality chart or set of local house cusps. This can supplement a natal or progressed analysis by showing areas of changed emphasis between the place of birth and the current place of residence. Some astrologers draw up a completely separate chart, while some just add the local cusps to the natal chart.
A locality chart is a chart set up as if the person had been born in their current place of residence at the instant at which they actually were born elsewhere. (If they have never moved from where they were born, there is no point in a locality chart.) Since the same instant of time as that of their actual birth is used, there is no change in planetary positions between natal and local charts. All that changes is the house cusps (which change because they depend on sidereal time which is dependent on location).
All one needs to do to get local cusps, then, is substitute the local latitude and longitude for the natal latitude and longitude. The rest of the procedure for erecting the chart is unchanged.
To get local cusps, one determines the sidereal time at the new location for the original Universal Time of birth. Keeping the UT constant insures that you really are using the same instant of birth. To get the new sidereal time, just make the longitude-time correction from the original Greenwich Sidereal Time to the longitude of the new location.
For me, this means converting 118 W 15 to time instead of my natal 110 W 58. 118° 15’ converts to 7 hours 53 minutes 0 seconds. My natal GST is 19:37:57. My sidereal time for a relocation to Los Angeles is: 19:37:57 - 7:53:0 = 11:44:57.
With this new sidereal time, look up house cusps for the latitude of the new location.
My local Midheaven comes out at 25 Virgo 53, my local Ascendant at 11 Sagittarius 45. All the cusps are obtained by interpolation using the new local sidereal time and the local latitude.