For the estimation of the longitude of Jalo (El Erg) a slightly different procedure was followed from that adopted for the other principal camps on the route. It will be noticed from the map that the general direction of the march from Jaghbub to Jalo was predominantly from east to west, instead of from north to south as in the rest of the journey. Consequently the observed latitudes do not afford so good a means of correcting the estimated distances in this stretch as elsewhere. But fortunately the observed latitude at Jalo does enable us to correct the previous estimation by Hassanein Bey (in 1920) of the distance of that place from Jedabia, and this combined with the then observed bearings would lead to one value for the longitude for Jalo, while if we can assume the accuracy of the estimated distances from Jaghbub to Jalo, we may use the observed latitude of the latter place to correct the bearings and thus arrive at another value for its longitude.
From careful consideration of all the available data, the two methods appear likely to be about equal in degree of approximation. The position at present accepted for Jedabia (Lat. 30° 48′ 10″ N., Long. 20° 13′ 30″ E.) is open to a little uncertainty,[2] and the bearings taken by Hassanein Bey on his former expedition are probably a little less accurate than those of the present one. On the other hand, the estimations of the distances from Jaghbub to Jalo, as gaged by the latitude-control of the other portions of the route of the present expedition, are remarkably close to the truth, while a uniform correction to his bearings of less than half a degree would swing his dead-reckoning position for Jalo on to its observed parallel of latitude.
For the longitude of Jalo on the map I have therefore taken the mean of the two longitudes found (1) by assuming Hassanein Bey’s bearings correct from Jedabia and correcting his distances by the latitudes, and (2) by assuming his distances from Jaghbub correct and using the observed latitudes to correct his bearings. The results are:
| ° | ′ | ″ | |
| (1) From Jedabia, long. of Jalo (El Erg) | 21 | 29 | 48 |
| (2) From Jaghbub “ “ “ | 21 | 26 | 19 |
| Adopted mean | 21 | 28 | 3 |
It may be remarked in passing that the result places Jalo almost precisely where it is shown on Rohlfs’s map of 1880.
As regards the longitudes adopted for the other principal camps along the route, the procedure has been as follows. The route was divided into the following nine sections between principal camps where latitudes had been observed: Jalo, El Harrash, Taj, Arkenu, Ouenat, Erdi, Agah, Enebah, Bao, Furawia; and the compass-traverse for each section was plotted on a scale of 1:500,000 from the recorded bearings and estimated distances. A true meridian was then drawn on each section from the mean of the observed compass-variations at its two ends, and the total difference of latitude of each section was scaled off and compared with the difference of latitude given by the latitude-observations. This comparison gave, of course, the average error of estimation of distance along each section, assuming the bearings correct. The results of the comparison for the various sections are tabulated below:
Correction to Estimated Distances
| Section of traverse | Plotted difference of latitude | True difference fromlatitude-observations | True-plotted diff. lat. | Correction to estimated distances |
|---|---|---|---|---|
| km. | km. | km. | % | |
| Jalo-El Harrash | 375.0 | 399.0 | + 24.0 | + 6.4 |
| El Harrash-Taj | 131.5 | 134.2 | + 2.7 | + 2.1 |
| Taj-Arkenu | 217.7 | 223.7 | + 6.0 | + 2.8 |
| Arkenu-Ouenat | 36.0 | 37.0 | + 1.0 | + 2.8 |
| Ouenat-Erdi | 369.0 | 363.2 | − 5.8 | − 1.6 |
| Erdi-Agah | 75.6 | 79.2 | + 3.6 | + 4.8 |
| Agah-Enebah | 57.0 | 57.5 | + 0.5 | + 0.9 |
| Enebah-Bao | 99.0 | 97.7 | − 1.3 | − 1.3 |
| Mean error of estimation of distances,per cent | 2.6 | |||
The average error of distance-estimation in each section of the route having been thus found, the next step was to scale off the various departures from the plotted traverses, correcting for the errors of distance-estimation, and then to convert the departures into differences of longitude. When this was done, the resulting total difference of longitude between Jalo and Furawia was 2° 25′ 55″. But assuming the true longitude of Jalo to be that found above, and the true longitude of Furawia to be as shown on sheet 53 D of the 1:250,000 Sudan survey map of 1921,[3] we have:
| ° | ′ | ″ | |
| Longitude of Jalo | 21 | 28 | 3 |
| Longitude of Furawia | 23 | 38 | 10 |
| Difference | 2 | 10 | 7 |