The 1852 Astronomical Point (Texas)

This is something that pertains to a project I'm working on at the moment. In 1852, in the course of surveying and mapping the boundary between the United States and Mexico, Major William Emory made astronomical observations to determine the position of a monument he placed on the U.S. side of the Rio Grande River, opposite the town in Mexico known as Presidio del Norte in the State of Chihuahua, presently called Ojinaga.

Major William Emory (at a later date)

Emory reported that from his observations on various stars for latitude and the Moon for longitude, he had determined the monument to be in the following position , the longitude being in relation to the Royal Observatory at Greenwich, England.

29° 34’ 07.13”N and 104° 24’ 45.3”W.

His determination of the latitude of the monument was based upon observations of 26 pairs of stars made with an instrument known as a zenith telescope on eight different nights between July 15 and August 18, 1852. The pairs were chosen to have one star North of the observer and one star South, at approximately equal altitude angles to cancel the effects of atmospheric refraction. From the zenith angles measured to each pair of stars on a particular night, a value for the latitude of Emory’s station was computed, with measurements to the same pair on different nights giving a check.

Being far inland and lacking a telegraph to use for a standard time signal, Major Emory determined the latitude by the method of lunar transits, observing the local times at which the limbs of the Moon passed or “transited” the meridian due South of his monument. His lunar observations were made on fifteen nights between July 26 and August 29, 1852 and from them the longitude of the station was computed by two separate methods which gave systematically different results.

The first method relied upon the Nautical Almanac published for that year to get the apparent position of the Moon needed for the computation. The second used lunar observations made at the Royal Observatory in Greenwich, England on the same nights to derive the longitude of the monument in Presidio west of Greenwich directly from the difference in the times of lunar transit noted at each place.

In the second half of the nineteenth century, when the public land around Presidio was surveyed into tracts for eventual settlement, Major Emory’s monument known as “the Astronomical Point” was used as a fundamental reference point by various surveyors laying out large blocks of land. This was evidently done on the theory that since it had a known latitude and longitude, Emory’s Astronomical Point could be placed upon a map and so, by measuring the bearings and distances of points from Emory’s monument, could also the lines and corners of tracts being surveyed out. Some large blocks of surveys covering hundreds of square miles were laid out mostly on paper with only bearings and distances from the Astronomical Point given to fix the positions of the hundreds of sections of land upon the ground.

In determining the locations of lands today that were originally located by surveys connected to the Astronomical Point, one task is determining where the Astronomical Point actually was. It had been a subject of confusion at various times. The National Geodetic Survey database reports that a monument designated as “RP 27 A” established by the International Boundary Commission in 1932 on the bank of Cibolo Creek was reputed to be in the same position as Major Emory’s 1852 Astronomical Point. The same records of the National Geodetic Survey also note that Boundary Commission monument “RP 27 A” was washed away in a flood in 1974, however. So it and presumably any remains of Emory’s monument upon which “RP 27 A” had been erected in 1932, now most likely lie tumbled somewhere in the floodplain of Cibolo Creek or piled among the material excavated from the creek when later flood control projects were constructed.

We know reasonably accurately where “RP 27 A” actually was. The National Geodetic Survey records report the following position for it (PID BB0040) as

29° 34’ 06.56783” N and 104° 23’ 18.08456” W
NAD83 (1993)

The NAD83 (1993) position of “RP 27 A” gives a very good idea of where that monument that supposedly was in the position of Emory’s Astronomical Point was. However, it doesn’t confirm that “RP 27 A” actually was in the same spot as the Astronomical Point.

Major Emory’s report gives one important clue. He reported that the position of a cathedral in Presidio del Norte, present day Ojinaga, Chihuahua, had been determined by triangulation in relation to his Astronomical Point and reported the following latitude and longitude for the cathedral:

29° 33’ 53.12” N , 104° 26’ 27.7” W.

That latitude and longitude would have been based upon Major Emory’s astronomical observations and, particularly the longitude might possibly be in error by a mile or more. However, the relative positions between the Astronomical Point and the cathedral should have been fairly well determined, particularly the azimuth of the line from the Astronomical Point to the church.

An inverse computation between Emory’s Astronomical Point and the Cathedral in Mexico using the latitudes and longitudes reported by Emory gives the following result:

AstroPoint to Cathedral: 261° 06’ 46” (Astronomic Azimuth), 2789.9m (Ellipsoidal distance)

If “RP 27 A” is in fact in approximately the position of Emory’s Astronomical Point, then where the cathedral in Mexico stood in 1852 at the time of Emory’s survey should lie at that azimuth and distance from the position of “RP 27 A”.

In fact, the position at that azimuth and distance from "RP 27 A" falls at the oldest church in Ojinaga, Mexico, what is known as “Templo de Nuestro Padre Jesus” on the West side of the main plaza.

Just as a rough check, the position of the center of the front of the present structure as shown in satellite imagery obtained from Terraserver, a commercial service, is:

29° 33’ 52.6” N and 104° 25’ 00.1” W (NAD83)

The inverse computation between “RP 27 A” and that position at the approximate center of the front of the Cathedral is : 261° 06’ 21” (Geodetic Azimuth), 2779.5m (Ellipsoidal distance).

The difference of 0°00’25” between the azimuth computed from what Emory reported and that estimated from satellite imagery amounts to only a difference of 0.33m (about 1 ft.) at a distance of 2779.5m. That is essentially perfect agreement, particularly considering that the position of the center of the front of the present cathedral taken from satellite imagery with 0.6m pixel resolution is likely to be in significantly greater error than 0.33m.

The difference between the distances of 2789.9m and 2779.5m respectively computed from Emory’s report and estimated from satellite imagery is probably not significant in that it is quite possibly the result of the triangulation referring to some point on the church other than the front of the facade. The present form of the structure is different than appears in early 20th century photos which themselves most likely do not exactly represent the structure as it was in 1852 when the triangulation survey was made.