Washington DC's Isometric Map Grid
In the image below the three segments of Penn Ave were first painted purple. Segment 1 connects Georgetown to the White House, 2 the WH and CB, and 3 the CB and the East Branch of the river. Next the northwest segment (1) was extended past the WH to the East Branch. As you can see the straightened avenue (in red) ends up in the same place (on the East Branch) as the current avenue does.
Since Maryland Ave (toward the Jeff Mml from the CB) mirrors Penn Ave on both sides of the Capitol, if the building had been further south E. Capitol Street along with Lincoln Square would be relocated with Penn and Maryland Avenues and the Jeff Mml.
This topographic image depicting the features of the 1791 DC map shows that the straight line CB position (blue rectangle) fell on the side of a steep hill (lines equal 10 feet of elevation each). I suggest that this is why the Capitol Building was moved to the north.
My isometric grid notion is modified by my straight line hypothesis which suggests that the CB (and part of the map plan including the Jeff Mml etc) was displaced to the north of it's ideal location for topographic reasons. The ideal location being on a straight Penn Ave so that the CB and the rest of the displaced points would have fallen on locations determined by the grid. But not everyone is in agreement with my assessment.
While pretty much everyone is in agreement that the distance between the points of the pentagon sets the east west scale in the map, there are two different views on how the north south scale is determined. I suggest that this is achieved by the spacing of the top of the triangle (junction of New Hampshire and 16th St), Scott Circle and the White House, as I have shown above. This view leads us to the conclusion that the Capitol Building was displaced to the north.
Here the grid is projected on the map. Pennsylvania connects the WH and CB, Maryland connects the CB and Jeff Mml. As you can see, Virginia Avenue matches the grid too, although very little else of the map does. As a matter of fact, none of the other diagonal streets match this grid.
Note the nodes on Penn Ave at the N-S grid lines.