[Requests] Comments on DGGS candidate standard 15-104r3

Kevin Sahr sahrk at sou.edu
Thu Feb 4 23:13:07 EST 2016


PART A

1. Evaluator:

Kevin Sahr
Professor
Department of Computer Science
Southern Oregon University

2. Submission:

Document Number: OGC 15-104r3
Name:OGC Discrete Global Grid System (DGGS) Core Standard

PART B-1

1. Requirement:

General

2. Implementation Specification Section number:

Throughout Section 5.

3. Criticality:

Editorial; major if accuracy is considered desirable in the Background
section.

4. Comments/justifications for changes:

The proposed standard states that “conventional coordinate reference
systems address the globe with a continuous field of points”. However, no
representation of geospatial location on a modern digital computer is
“continuous”. The conventional representations that are being alluded to
are most likely tuples of fixed-width floating point values, which are
every bit as discrete as a DGGS representation. This section should be
corrected to accurately reflect the reality of geospatial location
representation on digital computers.

DGGS have been proposed a number of times as a functional replacement for
traditional vector/point representations, including (and beginning with)
the work of Geoffrey Dutton that is cited in this proposal.

PART B-2

1. Requirement:

General

2. Implementation Specification Section number:

5.2 History

3. Criticality:

Editorial

4. Comments/justifications for changes:

The “first formalized discrete global grid” was not Dutton’s QTM, it was
the hexagonal DGGS created for atmospheric modeling specified in: Sadourny,
R.; Arakawa, A.; Mintz, Y. (1968). "Integration of the nondivergent
barotropic vorticity equation with an icosahedral-hexagonal grid for the
sphere". Monthly Weather Review 96 (6): 351–356.

Perhaps the intent is to detail the “first discrete global grid” after that
class of structures was identified and named. But that could not have been
Dutton’s QTM either, as Dutton developed his grid prior to 1997, when my
colleagues and I coined the term “discrete global grid”.

PART B-3

1. Requirement:

General

2. Implementation Specification Section number:

Throughout the document.

3. Criticality:

Major

4. Comments/justifications for changes:

The proposed standard specifies that DGGS should be “equal area”. There are
many important applications of DGGS for which equal area cells are not
optimal, including the aforementioned representation of vector location. It
seems odd that the standard discusses a number of useful DGGS that are not
equal area (e.g., Dutton’s QTM) but then goes on to make an unnecessarily
limiting requirement that effectively implies that those structures are not
“actually” DGGS. Perhaps this spurious requirement is being pushed by a
specific vendor who is attempting to gain a competitive advantage at the
expense of creating an objective standard?

Note that the equal area requirement, as detailed in this proposal, would
mean that hexagonal DGGS based on the ISEA projection (such as the ISEA3H
and ISEA4H DGGSs that were invented by my colleagues and myself) would not
“be DGGS”, because such DGGS’ contain 12 pentagons at each resolution with
only 5/6 the area of the hexagonal cells. Therefore the ISEA3H/4H would not
pass the stated conformance tests.

The proposal should either remove the equal area requirement or instead
state that it is a standard for “Equal Area DGGS” only.

PART B-4

1. Requirement:

General

2. Implementation Specification Section number:

6.2.5.1.

3. Criticality:

Major

4. Comments/justifications for changes:

The proposed standard specifies that cell “centroid is calculated as the
center of n-dimensional area of a DGGS cell”. Why is “n-dimensional”
specified, when it appears that only 2- or 3-dimensional areas would be
relevant? More importantly, this is not the only useful definition of DGGS
“centroid”. Depending on the application, the centroids might be calculated
on either the projection surface or directly on the surface of the sphere
or ellipsoid.
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