When you are doing a couples compatibility chart how accurate do you have to be on the birthtime and location? As for the location would it matter extremely if you were a few miles off on the location?
The time of each birth chart is necessary to determine the chart angles for a location and date. The location does not have to be as specific as distances of a hundred miles or so doesn't seem to greatly affect a chart in most cases.
Once you have two timed and approximate location charts you can do any/all of several things to examine compatibility, etc.
1) You can analyze the charts in terms of synastry. For example, one's Sun is trine the other's Moon, etc., and what does that mean? This approach to looking at a couple's interaction can provide quite a deep understanding if you can interprete charts well. For example, you look at "A's" chart and see how A approaches relationships in terms of 1st-7th axis, planets affecting that axis, and rulerships, etc. Within that construct, you then look at A's Sun and Moon relationship, Venus and Mars, Moon and Saturn, and Mercury and Jupiter relationships -- for Male-Female architype expression, Poise and physical expression, Family and social values, and ability to Communicate and grow thru interaction. Then, you do the same for "B". Finally, you place each of A's planets in B's chart to see how A reacts directly with B's various energies. All of this sounds complex, but it does give a very deep undertanding of how the couple will react within a relationship -- both individually and as a couple.
2) You can generate a composite chart wherein the mid-points of each factor in each chart is plotted as the composite factor. For example the Moon in one chart at Aries 10 degrees and the Moon in the other chart at Gemini 20 degrees would yield a mid-point of Taurus 15 degrees -- this is where the composite chart's Moon would be placed. You would then analyze this chart in terms of how the couple would work and live as a couple. These charts seem to work quite well as they represent very real "meeting points" between the individual planetary pairs. This type of chart does not analyze the individuals but the "coupleness" of the pair.
3) You can generate a mid-time chart (the exact term escapes me at the moment, as I don't use them) using the mid-point location and mid-point time between the two charts as the basis for defining the couples mode of interaction and cooperation. I've tried this approach a couple of times without being impressed by the results. That's not to say it isn't valid, it just wasn't valid for me when I tried it.
Hope this helps. Dave
Ok, what do I do if I can't get a birth time? Does that mean I can't do a compatibility chart at all?
You just use solar charts, each person's Sun as the Ascendant. Or, if you know one person's birth time then you do a synastry study from that person's point of view -- half an answer is better than none at all. Dave