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Natural Arch Identification

For accurate reference, as well as indexing purposes, every cataloged natural arch must be uniquely identified. Normally this is done by assigning each arch its own separate reference label. A standard for natural arch reference labels is proposed below, along with a set of guidelines for assigning them. However, before the standard is defined, the need for it is briefly discussed.

Reference Labels and Catalog Numbering

All serious natural arch catalogs published to date include reference labels for the arches they contain. Reference labels are frequently called catalog numbers. They should not be confused with the name(s) given to an arch. All of the various authors of natural arch catalogs have abandoned the use of names to uniquely identify arches. The reason for this can be found at the link "How do natural arches get named?" Of course, any names that exist for an arch are included in the information provided by these catalogs. But such names never supplant the catalog number (reference label) as the primary way of indexing the arches in the catalog.

Most authors have chosen to provide catalog numbers (reference labels) consisting of two parts – a designation for a geographical area and a separate number for each arch cataloged within that area. The format for these labels is usually some variant of A-N, where A designates the geographical area (it may be a number or group of letters) and N uniquely numbers each arch listed by the catalog in area A.

The geographical areas found in current natural arch catalogs are usually defined by political boundaries, e.g., the borders surrounding a country, a state, a county, a park, or some group of these. Other geographical criteria are occasionally used to define an area, e.g., a mountain range, plateau, or continent. In some catalogs, the second part of the reference label, N, orders the arches within area A in some way, e.g., by longitude, latitude, or significance (as defined by the catalog author). In other catalogs, N is assigned in an arbitrary sequence, e.g., the order correspondents reported the arches to the author. Not only does the method for sequencing N vary from author to author, it even varies within the work of the same author. In one major catalog, arches are ordered by significance in some areas, longitude or latitude in other areas, and arbitrarily in still others. This is a reflection of the fact that most catalogs have been published in parts over a period of several years.

Problems with Current Approaches

Although the A-N structure for reference labels might be considered a de facto standard, there are reasons why a new standard is needed. These are discussed below.

Catalog overlap. Natural arch catalogs by different authors have overlapping coverage. As a result, many natural arches have been assigned multiple reference labels, one from each catalog in which they are included. For example, Landscape Arch in Arches National Park is labeled as 2-2 in Vreeland's catalog (reference 1), SD-56 in Stevens/McCarrick (reference 2), and 1.1612 in Tom Van Bebber's World Arch Database. Although cross-references have been developed, it is clear that the A-N structure has resulted in different authors defining different areas and then numbering arches differently within those areas. This in turn creates potential confusion. A standard method for uniquely identifying natural arches that all catalogs followed would help avoid that confusion. The proposed standard is easily adopted by anyone wishing to develop or revise a natural arch catalog, or just contribute a single entry in a catalog.

Catalog maintenance. When things change, the A-N structure gets broken. Catalog maintenance becomes problematic. For example, consider the impact of a change in political boundaries that define an area within a catalog. This happens with surprising frequency. The boundaries of Arches National Park have changed several times since it was first created in the 1920's. Counties change regularly, too. Not only have there been several boundary adjustments, at least three new counties have been created in western US states in the past 20 years. Finally, compare the current borders of Balkan countries with those of 1990. Each time something like this happens, some arches that were in area A aren't anymore, and some that weren't, now are. Should these arches have their reference labels changed? If so, it's usually clear what the new A should be, but what number N should be assigned? Inserting a new arch into an ordered sequence could be difficult. Also, taking an arch out of a sequence will leave a gap. Should the remaining arches be renumbered to remove these gaps? Under the A-N structure, leaving the reference number unchanged might mean the arch is cataloged in the wrong area, the ordering of arches in an area might be incorrect, confusing gaps in catalog number sequences might be introduced, or all of the above. For this reason, it would be better to have a reference label that either never has to change, or when changes must be made, no maintenance issues result. This is a major advantage of the proposed standard reference label.

A Proposed Standard Reference Label for Natural Arches

The proposed standard for labeling natural arches is based on their locations within the Universal Transverse Mercator (UTM) system and the World Geodetic System of 1984 (WGS84) datum. The complete reference label for any natural arch in the world is its full UTM coordinates together with the prefix 'NABSQNO'. This prefix is an arbitrary character string used to distinguish the reference label from a location. Thus, reference labels have a format of NABSQNO Z-E-N, where Z is the UTM zone, E is the UTM easting, and N is the UTM northing. For example, the standard reference label for Landscape Arch is NABSQNO 12S-620979-4294446.

Comprehensive information about the UTM system and the WGS84 datum can be found at several websites such as Using the UTM Grid System and Using the UTM/MGRS map coordinate system. The reader is encouraged to become familiar with these before proceeding. It is especially important to understand what map datums are. A lack of understanding about map datums can lead to position discrepancies of hundreds of meters. For example, current USGS topographic maps give the UTM location of Landscape Arch as 12S-621040-4294243. That's because most US topographic maps use the North America Datum of 1927 (NAD27) rather than WGS84.

Advantages of the Proposed Standard

The location of a natural arch is its only property that is intrinsic, fixed, and not shared with any other arch. It is not assigned arbitrarily by a human observer, it does not change, and no two arches occupy the same exact space. Furthermore, for most arches, it can be determined easily and objectively by anyone with basic skills using a GPS receiver and topographic map. Thus, location is an ideal candidate for uniquely labeling and indexing arches. Since UTM is by far the easiest location system to use, the proposed standard is based on it.

The standard is based on WGS84 rather than NAD27 for several reasons. First and foremost is that it is a world-wide datum. NAD27 is only for North America. Natural arches on other continents could not be located correctly using NAD27. Also, WGS84 is the default datum used by Global Positioning System (GPS) receivers. Finally, the USGS is converting all future mapping efforts to use WGS84. TopoZone.com now provides map locations using WGS84 as the default option, and offers conversion between WGS84 and NAD27 for coordinates within the U.S. Converting between datums worldwide can also be done at this coordinate conversion site.

Remember also that the proposed standard is for the reference label only. It is not a standard for how catalogs should present location. Publishing a location for a North American arch using NAD27 for convenience in no way contradicts the use of a reference label that is based on WGS84. For that matter, the location could also be published using longitude and latitude rather than UTM coordinates.

If all natural arches were indexed by their UTM coordinates, they could be directly sorted by east-west or north-south position. Moreover, they could easily be grouped by area. For example, the natural arches within the boundaries of Arches National Park could be easily determined and listed. Furthermore, if those boundaries changed, the arches in the park could readily be re-determined and re-listed without changing the reference labels of the arches.

When a natural arch became newly cataloged, the Z-E-N (UTM) part of its reference label would fit easily 'between' the reference labels of those arches already cataloged. Should an arch be deleted from a catalog, striking its Z-E-N reference label would not leave an apparent gap in those remaining. Finally, if all catalogs referenced arches by their UTM coordinates, the arches that were listed in multiple catalogs would have the same reference label in all of them. Cross-referencing between these catalogs would not be necessary. Of course, it would still be desireable to have cross-references to older, historic catalog numbers.

Finally, having the character string 'NABSQNO' as a prefix enables reference numbers to be handled as such by computer applications, especially Internet search engines. To get a demonstration of this, go to a search engine such as Google and do a search on [Landscape Arch]. You will get back several hundred thousand sites, only a very few of which have anything to do with the natural arch we want information about. Now repeat the search with quotes to search for the exact phrase, e.g., on ["Landscape Arch"]. This reduces the returned pages to a few thousand, but still only a very few of these are on our subject.

Next, remove the quotes and add NABSQNO to the search, i.e., search on [NABSQNO Landscape Arch]. Only those few pages that pertain to or reference the natural arch are returned. Now imagine using the Internet to search on [NABSQNO Arizona], or [NABSQNO Algeria], or ["NABSQNO 12S"]. If we set up Internet pages about natural arches in accordance with the proposed standard, the first query would return all the pages referencing natural arches in Arizona. The second would return French arches. The third would find all the arches in the UTM zone 12S, including Landscape Arch. Finally, imagine searching on [NABSQNO 12S-620***-4294***]. In the future, when Internet search engines support searches on wild card characters (*) within a word, this search will return all the arches in the same square kilometer that Landscape Arch is in. Of course any square kilometer could be entered. Furthermore, this capability would be available without any special database software, just a browser and access to the Internet. The value of the standard reference label should now be very apparent.

Issues with the Proposed Standard

There are two possible concerns about using UTM coordinates as the reference label for natural arches. The first is convenience. NABSQNO Z-E-N is much longer than any reference label based on A-N. The second is accuracy. How is the Z-E-N determined? Where on the arch is it measured? And just how accurate is it really? Let's discuss these in turn.

A 23-character reference label is certainly less convenient than the 4 to 7 characters that the A-N format usually requires. Fortunately, in practice NABSQNO Z-E-N will be mainly used by computers. People will still use names and shorter references. But the standard gives computers what they need for precise and rigorous identification. Of course, it can be used for the same purpose by people when they want. A key point is that the standard reference label does not prohibit the use of shorter, more convenient labels as supplemental reference aids. For example, if a standard reference label were added to all the natural arches in Vreeland's catalog, there would be no need to delete his original catalog numbers. Indeed, these should be preserved for historical reference. The Stevens/McCarrick catalog adopted the practice of attaching an English name to all of its arches for the convenience of people. Adopting the proposed standard reference label does not require that such practice be discontinued.

Now let's consider questions of accuracy. First, how is the Z-E-N determined? Currently, the best readily available method is to use a combination of a GPS receiver and a topographic map to determine the location of the arch as accurately as possible, and then to transform that location as needed to the WGS84 datum. As with any method, inaccuracies creep into the result. We consider the impact of these inaccuracies in the discussion below.

Where on the arch is the Z-E-N (location) measured? Natural arches are not points. They have horizontal extent. But reported locations are points. What convention should be followed for picking a point on an arch where its location measurement is taken? Ideally, the convention should be to report the location of the arch's center point as the location of the entire arch. Unfortunately, it is probably impossible to exactly define a center point for every type, shape, and size of natural arch. Even if an exact definition existed, it would be very difficult for field investigators to correctly apply it every time. Consequently, there will always be a spread or imprecision in the location of arches that is introduced by their horizontal extent. This spread can be estimated to be about 10% of the horizontal extent of an arch. For the very largest arches, this can be as much as 10 meters. A 10 meter inaccuracy means that the last number in both the easting and the northing are pretty meaningless.

How accurate are location measurements? With a typical GPS receiver and map it is usually possible to pinpoint your location, or the location of a point that is visible to you, with a precision of about 10 meters. Thus, if several independent observers measure the easting and northing of the exact same point and report its location to the nearest meter, even if they are all using similar GPS receivers and the same topographic map, they will report several different values. If the observers take the measurements correctly, almost all of their reported locations will be within about 10 meters of the average of the measurements. This average is the best estimate of the actual location of the point. The 10 meter spread is an indication of how accurate a location can be determined when no mistakes are made. We'll look at the consequences of mistakes in a bit. For now, note again the implication that the least significant digit of the easting and the northing are not really meaningful.

But so what? Do we really need an accuracy of better than 10 meters for the location? In the vast majority of cases, no. If you are told that an arch is at a certain point, and you draw a circle around that point with a radius of 10 meters, odds are you will find only one arch inside it. Even if it is a little bit off, the Z-E-N has uniquely identified the arch. That's all we need. It's kind of like saying that the Z-E-N uniquely identifies the one arch that is close to that location.

What about when there is more than one arch in the circle, i.e., when we are trying to identify a closely packed cluster of small arches. Even this case is not usually much of a problem. The errors that show up in the measured location of one arch in a small cluster will impact the measured locations of the other arches in that cluster in the same way. For example, the measured locations might all be wrong by 10 meters in the northeast direction, but their locations relative to each other will be preserved. If we are only talking about 2 or 3 arches in the cluster, identification will not be a problem. In the rare cases where many very small arches are clustered in a 10 meter circle, unique identification using the Z-E-N is not practical. But in those cases there is probably little value in unique identification. Instead, it is probably more valuable to identify the cluster as a single feature. Of course, that can be done using the Z-E-N for the cluster.

Of course, observers do make mistakes and incorrect location measurements certainly have been published on occasion. Obviously, when this happens an error of unknown magnitude is introduced in the location, and hence in a reference label derived from that location. Normally, mistakes of this nature are eventually discovered and corrections made to the published location. But what about the reference label? Should it change too? Yes, clearly it would have to. But, perhaps surprisingly, this does not create much of a maintenance problem. One could look at the correction as the simultaneous deletion of an arch that never existed (one never did exist at the erroneous location) with the addition of a new arch (at the correct location). When any catalogs that included the incorrect reference label got updated, they would likely include a page that listed the incorrect Z-E-N along with a note about the error and deletion. Not much different from how an updated catalog might treat an arch that had collapsed. It would state that the incorrect Z-E-N had been used in the past as a reference label, but that there is not an arch at that location.

Guidelines for Using the Proposed Standard

Hopefully, the above discussion is convincing that the UTM location (Z-E-N) method for identifying arches is a better way, and that it deserves to become a standard. It may not be perfect, but it is clearly superior to the other approaches used to date. Before it can become a standard, however, guidelines must be created and adopted to describe how these reference labels are to be assigned and managed within the context of natural arch cataloging efforts. A preliminary set of guidelines follows.

If the guidelines below are followed by everyone who assigns a standard reference label to an arch, the full benefits of the standard will be realized while any accuracy problems will be minimized. Of course, one of those benefits is the continuing development of an Internet based, on-line database of natural arches. The guidelines below should also be kept in mind when using the standard to query that database. Note that, once locations accurate to 1 meter are easily obtained in the field, some of these guidelines will be unnecessary and will be revised. Note also that these guidelines apply to the reference labels only, not to published locations.

  1. Place the reference label prefix (NABSQNO) only on web pages intended to convey documentation about an arch, e.g., photographs, location, type determination, names, history, dimension measurements or estimates, or any other standard attributes of the arch. Do not include the prefix on a web page that merely refers to the arch, e.g., an index to other pages that do provide documentation. In these cases, refer to the arch with just the prefix N. For example, an index page might include Landscape Arch in a list with the reference N 12S-620979-4294446.

  2. Include, as appropriate, the country, state, province, and/or county in which the arch is located somewhere on the page. Include any names that are associated with the arch on the page.

  3. Only have one arch (and one reference number) per page unless the multiple arches shown are in a cluster that you want to document as a single feature. This will avoid the search engines mixing data from one arch with the reference label for another arch.

  4. Only assign a full reference label if you are certain of the location to 10 meter accuracy. If you know it to less accuracy, you may assign a temporary label that reflects this fact. Substitute the character 'X' for the unknown digits of the easting and northing. For example, if you knew the location of Landscape Arch to only 100 meter accuracy, you might assign a temporary reference label of NABSQNO 12S-6209XX-42944XX. If you knew it only to 10 kilometer accuracy, you might assign NABSQNO 12S-62XXXX-429XXXX. However, if you only know the UTM zone for an arch, follow the zone with the abbreviation 'TBD' for both easting and northing. For example, Landscape Arch might be assigned the temporary label NABSQNO 12S-TBD-TBD. TBD stands for 'to be determined.' Obviously, such temporary labels do not uniquely identify the arch, they are merely place holders so that arches with poorly determined locations can be partially cataloged.

  5. Before assigning a reference label to an arch, ALWAYS check to make sure that one has not already been assigned. NABS endeavors to maintain a complete set, so it is best to check with us prior to making an assignment. This will avoid multiple reference labels for the same arch. Inquiries of this nature, and any reference numbers you assign, should be sent to info@naturalarches.org.

  6. Always assign the easting and northing parts of the reference label to the nearest meter, even though the location is only known to 10 meter accuracy. It should be understood that the last digit of the easting and northing in a full reference label are arbitrary.

  7. Always assign the reference label based upon the WGS84 datum.

  8. Once a full NABSQNO Z-E-N reference label has been assigned to an arch, it should only be changed if: (1) it is discovered that the location it is based upon is wrong by 100 meters or more, or (2) it is necessary to avoid confusing it with another, close-by arch. For example, if the location of Landscape Arch were found to be actually 40 meters to the north of the location that its reference label is based upon, the reference label for Landscape Arch should not be changed, even though the published catalog location is refined. Remember that a 40 meter shift is less than the size of this arch.