Six-digit MPI entries in WWII Eighth Air Force field orders

Chuck Varney
21 Apr 18


Background

In 2013 Tom Harrison published a book, “Kassel”, that purported to explain how a navigational error occurred that caused the Eighth Air Force’s 445th Bomb Group to suffer great loss of aircraft and crews on a 27 September 1944 bombing mission to Kassel, Germany. He built his explanation around a claim that the first six digits of a field order MPI entry of the form 070058/14 represented a pair of GEE-H distances, and that MPI meant “mean point of intersect” (rather than the correct “mean point of impact”). He then took the asserted GEE-H distances and wove them into a story, which—for him—explained the cause of the navigational error. I found his initial claim, and all his subsequent explanations of equipment operation that enabled the navigation error, to be demonstrably wrong.

I was left wondering how to properly decipher an MPI entry like 070058/14. I suspected that 070 and 058 were grid map coordinates and speculated that “/14” was a chart, sheet, or book reference related to the grid. I searched for, but failed to find, field orders for other Eighth Air Force missions that might shed some light.

April 2018

Kassel revisted

 I revisited the MPI 070058/14 entry, looking for grid maps used in the European Theater.  Thierry Arsicaud’s echodelta.net site provides a Nord de Guerre zone grid map that covers Germany, and a coordinate translator that enables conversion from Nord de Guerre coordinates to geographic coordinates (latitude and longitude). I used them to determine that Kassel, Germany lies in Nord de Guerre grid rC.

The Target and MPI columns from division level Field Order No. 470 are given in Figure 1.

I wanted to test whether the 6-digit MPI's might be Nord de Guerre grid coordinates with the rC prefix removed. I used  the coordinate translator to find the geographic coordinates corresponding to rC124021, rC058072, rC069067, and rC070058.

Figure 2. shows target area and MPI locations on Google Earth, assuming the MPI entries are Nord de Guerre coordinates.





Figure 1
. Excerpt from 2nd Bomb Division Field Order No. 470 for the Kassel 27 September 1944 mission





Figure 2. Plot of the 2nd Bomb Division target area (Henschel and Son Works, in red rectangle) and MPI's (red diamonds) for the
               27 September 1944 Kassel mission


The plotted MPI results fall well outside the target area, so the field order MPI's are not simply Nord de Guerre grid coordinates with the rC prefix removed.


Eighth Air Force 24 December 1944 missions

I searched for other Eighth Air Force field order examples in the hopes that they might provide further insight into the MPI entries. I found Joost de Raaf’s detailed documentation of the Eighth Air Force missions flown on 24 December 1944 by the 1st, 2nd, and 3rd bomb divisions. In it, he has collected 86 pages of field orders, supplements, and annexes, distributed among the three bomb divisions. I extracted the MPI portion of division level field orders as shown in Figures 3., 5., and 7., below. I used the same process as described in Kassel revisited, above, to determine appropriate Nord de Guerre grids, and to plot MPI's and target areas on Google Earth. For the intended primary targets I used this Eighth Air Force Historical Society page, where 1. lists six 3rd Bomb Division targets, 2. lists five 1st Bomb Division targets, and 3. lists fourteen 2nd Bomb Division targets.

1st Bomb Division, 24 December 1944


The Target and MPI columns from division level Field Order No. 587 are given in Figure 3.

Figure 4. shows target area and MPI locations on Google Earth, assuming the MPI entries are Nord de Guerre coordinates.




Figure 3. Excerpt from 1st Bomb Division Field Order No. 587 for 24 December 1944



Figure 4. Plot of 1st Bomb Division target locations (yellow circles) and MPI’s (red diamonds).

Only five of the 15  MPI's of Figure 3. are plotted in Figure 4. because 14 of them fall within the white rectangle and would be unreadable if plotted. The four shown in the rectangle mark the most northern, southern, eastern, and western of the 14.

Again, like the Kassel example, the plotted results do not map into the target areas, so the given MPI's are not Nord de Guerre coordinates with the appropriate prefix (wH) removed.



2nd Bomb Division, 24 December 1944


The Target and MPI columns from division level Field Order No. 554 are given in Figure 5.

Figure 6. shows target area and MPI locations on Google Earth, assuming the MPI entries are Nord de Guerre coordinates.

 

Figure 5. Excerpt from 2nd Bomb Division Field Order No. 554 for 24 December 1944




Figure 6.
Plot of 2nd Bomb Division target locations (yellow circles) and MPI’s (red diamonds).



Twelve of the 13 plotted MPI's fall on the given target locations--and the 13th is close to one, so in this case the 6-digit MPI's with the appropriate prefixes (wL and wF) work as Nord de Guerre coordinates. There is a reason.


The 13 2BD 6-digit MPI's have a TARGET column entry of GSGS4416 with a T1 or S1 in the next column. Internet searches revealed that G.S.G.S. 4416 was a British War Office map series covering central Europe at a scale of 1:100,000 and that T.1. and S.1. were sheets in that Map series. G.S.G.S. is an abbreviation for General Staff, Geographic Section.  The grid lines on these sheets (follow the directions on this McMaster University page to view them) are Nord de Guerre grid lines.

Figure 6a. attempts to show how Nord de Guerre grid blocks wF and wL relate to G.S.G.S. 4416 Sheets R1, S1, and T1 in terms of geographic extent. For simplicity, the rectangles present angular extent as if the earth were flat.








Figure 6a. Relationship between G.S.G.S. 4416 Sheets R1, S1, and T1 and Nord de Guerre grid blocks wF and wL



3rd Bomb Division, 24 December 1944

The Target and MPI columns from division level Field Order No. 526 are given in Figure 7.

Figure 8. shows target area and MPI locations on Google Earth, assuming the MPI entries are Nord de Guerre coordinates.




Figure 7. Excerpt from 3rd Bomb Division Field Order No. 526 for 24 December 1944



Figure 8. Plot of 3rd Bomb Division target locations (yellow circles) and MPI's (red diamonds)

The five MPI's given in Figure 7. plot as a group to the south and west of the target locations. Four have suffix "/2"; one has "/4". Lke the Kassel and 1st Bomb Division examples, the plotted results do not map into the target areas, so the given MPI's are not Nord de Guerre coordinates with the appropriate prefix (wM) removed.

Conclusion


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