In vedic astrology what is difference between bhava madhya and rashi madhya and what is their signif

[Astrovalley]

Rasi Chart assumes that all the Houses are of equal in length of 30 degrees each, and the Rashi madhya is 15 degrees in that Sign. 
Whereas in the Bhava Chalita Chart as it is called is calculated as per the Sripathi Padhati in Vedic Astrology. Bhava Madhya is not 15 degrees as in Rashi , but differs as per the Place of Birth and calculated taking into account the Ascendant, 10th House and the 4th House and deviding the difference into 3 parts etc. Thae mid point or Bhava Madhya is important in prediction.

For example if the Ascendant is 17 degrees Saggitarius, the Lagna Bhava starts from 2 Degrees Saggittarius to 2 Degrees Capricorn. 
The Bhava Madhya is 17 degrees Saggittarius, which is the Ascendant. The starting and ending points is called Bhava Sandhi. 
However, the aspects of planets on other planets are always studied from rashi chart. Similarly exaltation/debilitation of planets is again studied only from the rashi chart. Rasi Chart, talks about the relative positions of planet for identifying the nature of the planet (benefic/ malefic etc.) and relationship with other planets. 

For example, a planet 6th, 8th, or 12th from lagna is not good for lagna. For example if a planet is in 8th in lagna kundli but in 7th in Bhava kundli, it means that you will get the result of 7th house during the dasa period of that planet. Due to being in 8th in lagna chart, the result may be malefic, so it shows problem in marriage.