Time-dependent datum problems

Technical shortcuts taken in mapping systems, which ignore tectonic plate movements, are causing meter level map misalignments, most notably in web-mapping. It is caused by a cluster of interrelated problems, which we attempt to comprehensively catalogue in this article. For a more concise summary, look here.

Caution: This article is full of acronyms and complex geospatial mapping concepts. It is primarily aimed at technical experts in this domain. It provides a snapshot in time, collated from a range of people. It is expected to have inaccuracies and not be up to date with latest thinking.

Main problems

Web-mapping’s illusion of accuracy

• To date, web-mapping has provided an illusion of accuracy; all map layers have typically been aligned; but they have all been aligned to the WGS 84 epoch (and tectonic plate location) associated with the regional datum. This is being exposed as misaligned maps where a nation creates new national datum(s) as part of datum modernisation programs.
• Australia is experiencing this problem now, due to an update of the nation datum from GDA94 to GDA2020.

WGS 84 datum

• WGS 84 Web Mercator is the defacto standard used in web-mapping.
• WGS 84 has inherent ambiguity which in practice introduces low accuracy:
• It has multiple realizations (it has been re-defined six times to date).
• In web-mapping, the dynamic WGS 84 datum is being used as if it were a static.
• In contrast, GPS positioning delivers point positions in the WGS 84 dynamic datum at the ‘current’ epoch (date).
• Different epochs of WGS 84 have been adopted, in practice, in different regions of the world.
• It uses step-wise annual updates rather than being dynamic at any time-scale.
• … and more
• Static tiled basemaps around the world provides a major barrier to moving away from the WGS 84 Web Mercator projection, as all existing maps would need to be reprojected, or face misalignment with new datums.
• Adopting a new web-mapping datum would be costly, requiring extensive retiling of all the world’s tiled maps and stored datasets.

Spatial Standards

• To accurately describe coordinates, spatial standards describing position in a dynamic environment must accurately describe:
• The coordinates (e.g. Latitude, Longitude, Height),
• The datum (reference frame) and
• The epoch (date) at which the coordinates are expressed. (This is often different to the epoch at which coordinate was first observed!)
• Epoch is implicitly defined for a static datum (e.g. 1994.0 in GDA94) and needs to be explicitly defined in a dynamic (time-dependent) datum (e.g. ITRF1992@1994.0). Unfortunately, epoch is not incorporated in legacy standards, so updating the way in which dynamic datums are defined and applied must be addressed in the full stack of OGC standards.
• Even the most widely used of OGC standards, like the Web Map Service (WMS) standard, do not account for time dependence when selecting a dynamic Coordinate Reference System (CRS).
• File formats such as KML and GeoJSON only support storing coordinates in WGS 84. This needs to be considered if addressing future accuracy requirements.

Communication challenges

Collaborating to solve mapping challenges is hindered by communicating a difficult problem, with complex terminology, to a diverse audience. For example:

• There is a general lack of use and understanding of ‘accuracy’, and how the choice of transformation and projection can affect accuracy.
• Authoritative documents use terms differently in different documents, or use multiple terms for similar concepts. For example:
•  “datum”, “reference frame”, and “CRS”, all have similar meanings.
• There are several different concepts of epoch which are required to accurately describe coordinates in a dynamic datum. For example, a coordinate can be observed at epoch 2017.21, but recorded or expressed at epoch 2018.5 in a reference frame of ITRF2014.

Action Plan

Urgency

There is a need for urgency because:

• Australia has 1.8 metre map misalignments now.
• North America plans to introduce new datums in 2022.
• High-precision mass-market positioning is just around the corner.

Technically

We need to:

• Solve web-mapping dynamic/static misalignments.
• Advance time-dependent mapping.
• Update standards.
• Roll into software.
• Improve world mapping practices and user outcomes.

Community mobilisation

While we have technical challenges, we have a harder outreach and community mobilisation challenge. Success depends on collaboratively bringing software, data and geospatial communities with us. We need core buy-in from:

• Leading tiled map providers,
• Spatial data providers,
• Software venders, and
• Government mapping agencies.

We need clear messaging that Julie, the GIS officer, and Joe, the graduate software developer, can use to explain the datum modernisation story.

Web-mapping business requirements

Let’s revisit the business requirements of accurate web-mapping to help us assess the viability of current and proposed solutions.

Legend for current support:

• Supported
• Partially supported
• Not supported

Geospatial requirements:

1. Support high accuracy mapping.
2. Account for time dependence resulting from tectonic plate movement.
3. Provide accurate map alignment when displaying map layers from disparate sources.
4. Support calculation, publishing and application of accuracy metadata.
5. Datasets must have a nominated datum (reference frame) and epoch (date). Epoch is implicitly defined for a static datum and needs to be explicitly defined in a time-dependent datum.

Usability requirements:

6. Datasets must be able to be transformed between datums and epochs.
7. Software applications shall continue to be responsive and performant for users, including for low-spec clients, such as browsers and mobile apps.
8. Web-mapping services shall continue to scale efficiently to support multiple users.
9. Users shall continue to be able to save map vector data as static files (such as KML, GML, GeoJSON), and render at a later point in time.
10. Accuracy information should be embedded in decision workflows.
11. Spatial expertise required to be learned by software implementers should be minimised.

Software Implementation requirements for web-mapping:

12. Tiled web-mapping shall continue to be supported to address performance and scaling. In practice, this freezes maps in time.

Derived CRS requirements:

13. Adopt a CRS, datum and projection for publishing map layers with the following characteristics:
1. Defined accurately.
2. Receives accurate transformations from key source CRSs (in particular, from national/regional datums).
3. Aligned with a conventional fixed epoch (date).
4. Receives coordinates converted from other epochs.
5. Applicable at global and local scales.
6. Near-universally adopted within web-mapping contexts.

Main problems

Web-mapping’s illusion of accuracy

• To date, web-mapping has provided an illusion of accuracy; all map layers have typically been aligned; but they have all been aligned to the WGS 84 epoch (and tectonic plate location) associated with the regional datum. This is being exposed as misaligned maps where a nation creates new national datum(s) as part of datum modernisation programs.
• Australia is experiencing this problem now, due to an update of the nation datum from GDA94 to GDA2020.

WGS 84 datum

• WGS 84 Web Mercator is the defacto standard used in web-mapping.
• WGS 84 has inherent ambiguity which in practice introduces low accuracy:
• It has multiple realizations (it has been re-defined six times to date).
• In web-mapping, the dynamic WGS 84 datum is being used as if it were a static.
• In contrast, GPS positioning delivers point positions in the WGS 84 dynamic datum at the ‘current’ epoch (date).
• Different epochs of WGS 84 have been adopted, in practice, in different regions of the world.
• It uses step-wise annual updates rather than being dynamic at any time-scale.
• … and more
• Static tiled basemaps around the world provides a major barrier to moving away from the WGS 84 Web Mercator projection, as all existing maps would need to be reprojected, or face misalignment with new datums.
• Adopting a new web-mapping datum would be costly, requiring extensive retiling of all the world’s tiled maps and stored datasets.

Spatial Standards

• To accurately describe coordinates, spatial standards describing position in a dynamic environment must accurately describe:
• The coordinates (e.g. Latitude, Longitude, Height),
• The datum (reference frame) and
• The epoch (date) at which the coordinates are expressed. (This is often different to the epoch at which coordinate was first observed!)
• Epoch is implicitly defined for a static datum (e.g. 1994.0 in GDA94) and needs to be explicitly defined in a dynamic (time-dependent) datum (e.g. ITRF1992@1994.0). Unfortunately, epoch is not incorporated in legacy standards, so updating the way in which dynamic datums are defined and applied must be addressed in the full stack of OGC standards.
• Even the most widely used of OGC standards, like the Web Map Service (WMS) standard, do not account for time dependence when selecting a dynamic Coordinate Reference System (CRS).
• File formats such as KML and GeoJSON only support storing coordinates in WGS 84. This needs to be considered if addressing future accuracy requirements.

Communication challenges

Collaborating to solve mapping challenges is hindered by communicating a difficult problem, with complex terminology, to a diverse audience. For example:

• There is a general lack of use and understanding of ‘accuracy’, and how the choice of transformation and projection can affect accuracy.
• Authoritative documents use terms differently in different documents, or use multiple terms for similar concepts. For example:
• “datum”, “reference frame”, and “CRS”, all have similar meanings.
• There are several different concepts of epoch which are required to accurately describe coordinates in a dynamic datum. For example, a coordinate can be observed at epoch 2017.21, but recorded or expressed at epoch 2018.5 in a reference frame of ITRF2014.

Main options

Option 1: Do Nothing - Low accuracy web mapping

• Accept poor accuracy and misaligned maps in web-mapping.
• This is unacceptable for Australia.

Option 2: Call a spade a spade

• By convention, acknowledge that WGS 84 is currently being used as an accurate static datum in web-mapping.
• This will likely be accompanied by defining new EPSG code(s) to recognise the currently implemented datum.
• We would need to acknowledge that WGS 84, as used by the GPS system, is a different datum entirely.
• In Australia, we would acknowledge that WGS 84 is aligned with our old GDA94 datum.

Option 3: Adopt a common fixed epoch for web mapping

• Adopt a common fixed epoch to facilitate static tiled maps of the world. This epoch might be periodically re-baselined into pre-planned “epochs of convenience”.
• (Currently, maps are tiled into different epochs in different regions.)
• Note: This would involve retiling all the world’s basemaps to the new common epoch.
• Note: if committing to retiling all maps, we can consider additional improvements to map tiles, such as moving away from the less accurate spherical assumptions of the earth used by WGS 84 Web Mercator.

Option 4: Adopt dynamic datum

• Adopt a conventionally agreed datum which is explicitly dynamic and internationally recognised, such as ITRF. This will allow new data to be more easily moved through time back to the common epoch.
• Note: this would involve retiling all the world’s basemaps to the new datum.

Option 5: Use client based datum/epoch conversion tools

• Allow users to obtain coordinates in (any) desired datum and/or epoch by transforming (x,y) mouse coordinates on screen, rather than expressing and storing underlying base-maps in multiple datums. (Move Mohammad to the mountain rather than moving the mountain to Mohammad.)

Update spatial standards

• Almost all options will require updates to standards to cater for time-dependence. For example:

• Web Services such as WMS, WFS, etc., which accept a dynamic CRS as a parameter, will additionally need to accept a CRS epoch field.
• Data transfer formats such as KML and GeoJSON which only store coordinates in the WGS 84 datum will need to be extended to support more accurate datum(s).

Clear messaging

• Investing in clear messaging should help with communication challenges.
• We should simplify developer and user experiences by abstracting complicated concepts behind simple implementations.

Action Plan

Urgency

There is a need for urgency because:

• Australia has 1.8 metre map misalignments now.
• North America plans to introduce new datums in 2022.
• High-precision mass-market positioning is just around the corner.

Technically

We need to:

• Solve web-mapping dynamic/static misalignments.
• Advance time-dependent mapping.
• Update standards.
• Roll into software.
• Improve world mapping practices and user outcomes.

Community mobilisation

While we have technical challenges, we have a harder outreach and community mobilisation challenge. Success depends on collaboratively bringing software, data and geospatial communities with us. We need core buy-in from:

• Leading tiled map providers,
• Spatial data providers,
• Software venders, and
• Government mapping agencies.

We need clear messaging that Julie, the GIS officer, and Joe, the graduate software developer, can use to explain the datum modernisation story.

Detailed problem list

In this section we provide a “shopping list” of problems which hinder the adoption of high-accuracy mapping. It builds from experiences in implementing Australia’s datum modernisation program and ties into international challenges. It touches on time-dependence, standards, web-mapping, and map misalignment challenges.

1. WGS 84 Datum ambiguity

As WGS 84 is the defacto standard used in web-mapping, it is worth understanding the relationship between projection, datum and CRS:

• The WGS 84 Web Mercator projection (EPSG::3857), technically called the Pseudo Mercator projection, is based on
• The WGS 84 Coordinate Reference System (EPSG::4326), which is based on
• The WGS 84 Datum ensemble (EPSG::6326), which is an ensemble of
• Six WGS 84 Datum realizations.

Figure: Basis of WGS 84 from EPSG Registry 

As explained by Roger Lott, the formal WGS 84 datum is not unique; it has been updated six times to date. Each update (called a realization) refines the datum’s alignment with the earth to account for improved measurements of the earth’s shape (which is different to tectonic plate movement). Each WGS 84 formal realization is dynamic. I.e. coordinates change with time. To be unambiguous, coordinate metadata needs to include coordinate epoch (date).

Problem 1.1 Non-unique datum:

The WGS 84 datum ensemble (EPSG::6326), represents any or all of the formal WGS 84 realizations, without distinction. It should be considered to be static, with metre level uncertainty.

Problem 1.2 WGS 84 coordinates are stepwise updated

Figure: Stepwise jumps in WGS 84

WGS 84 coordinates for ground stations and satellite ephemerides (and therefore the datum definition) are re-computed each year, mid-year. WGS 84 actually moves in a step-wise fashion. If this is not taken into account, then positioning problems can result. (Refer to ISO 19161-1.)

2. Modernising OGC Standards

To accurately describe coordinates, they must include the coordinates, a datum (reference frame) and epoch (date). Epoch is implicitly defined for a static datum and needs to be explicitly defined in a time-dependent datum.

The base OGC and ISO standards document, OGC Abstract Specification Topic 2: Referencing by coordinates, introduces modern geodetic time-dependent concepts, including:

• Extension to describe dynamic geodetic reference frames.
• “Datum ensembles” which allows grouping of related realizations of a reference frame.
• In a dynamic CRS, the coordinate epoch is stored as an attribute of a set of coordinates, it is not part of the CRS. (A set of coordinates could be points, features, or a dataset.)

Problem 2.1: Coordinates in Dynamic CRS are ambiguous

To date, coordinates which have been stored in a dynamic CRS have been ambiguous, as the observation epoch typically has not been recorded, and epoch is not stored in the CRS either.

This is proposed to be addressed by including epoch with coordinates when using a dynamic CRS.

Problem 2.2 Implementation challenges with time-dependent standards

• We will need to consider how new time-dependent coordinates can be introduced without breaking backward compatibility with existing standards, software, and datasets.
• There is likely to be performance and implementability implications depending on whether epoch is tied to the level of a point, feature, or dataset.

These should be considered before continuing with standard definitions.

Problem 2.3: OGC standards which don’t address time dependence

Core OGC standards still don’t address time-dependent coordinate referencing. This includes standards such as the GML format, WMS, WFS and so on.

Problem 2.4 High impact to update software to support time dependence

Changing the core coordinate definitions to support time dependence is likely to have a significant impact on software implementations, and stored datasets.

Problem 2.5: KML standard uses WGS 84

The OGC KML 2.3 standard, initially defined by Google, is used widely for storing map data. By definition, KML stores coordinates in the ambiguous WGS 84 CRS (EPSG::4987), and doesn’t record epoch (date), resulting in dataset inaccuracies.

Problem 2.6: GeoJSON standard uses WGS 84

The current GeoJSON standard (dated August 2016) states:

The coordinate reference system for all GeoJSON coordinates is a geographic coordinate reference system, using the World Geodetic System 1984 (WGS 84) 

WGS 84 is not explicitly defined (by an EPSG code or similar) and could be any of the WGS 84 realisations. Likewise, epoch (date) is not recorded, resulting in further dataset inaccuracies.

Problem 2.7 Non-standard GeoJSON in use

While the GeoJSON standard only allows a CRS of WGS 84, “in the wild” implementations appear to allow implementation of different CRS values (as per earlier drafts from April 2006 of the GeoJSON standard). For instance, the OpenLayers browser based client supports a GeoJSON CRS attribute. See example:

Problem 2.8 Inconsistent reference to CRS

As explained in a Request for Change for content negotiation by CRS, inconsistency in referring to CRS is causing implementation problems:

Currently, many GeoJSON API based implementations support communication of CRS. The GeoJSON standard does not provide a standardized method of negotiating CRS. As a result, each implementation varies with respect to both how a CRS is requested, as well as the method used to communicate which CRS was used to locate the data returned. Most, if not all, implementations use the body of the request or response for communication.
There are three issues with this implementation option. Firstly, the name of the parameters used in requests and responses vary between implementations. For example, the variation of terms used to specify the name of the parameter include, but are not limited to, "SRS" (the acronym for Spatial Reference System) and "CRS". Secondly, the semantics of the value used in requests and responses also varies between implementations. For example, the variation of terms used to specify the name of the value for the World Geodetic System of 1984 include, but are not limited to, "WGS84", "4326", and "EPSG:4326". Finally, the value returned by a service does not necessarily support the ability to dereference the CRS.

3. Publishing static maps using a dynamic datum

Organisations have been inadvertently freezing dynamic maps in time when the maps are published. This prevents the feature’s coordinates from changing over time as should happen for coordinates stored in a dynamic datum.

Problem 3.1: Tiling dynamic datum maps

When maps are tiled, they are frozen in time. As the defacto standard for tiled maps is the dynamic WGS 84 Web Mercator projection, we are creating a static/dynamic map misalignment problem.

Problem 3.2: Publishing and distributing dynamic datum maps

Some organisations have been publishing map data in the ambiguous WGS 84 datum. Also, most web services are configured to allow users to download maps in various datums, including WGS 84. When downloaded, these maps are frozen in time, creating the static/dynamic map misalignment problem. The problem is often exacerbated by technology and data providers who encourage use of WGS 84 to facilitate interoperability with map tiles.

4. Web-mapping has been topologically aligned, but not accurate

To date, web-mapping has typically appeared more accurate than the metre level accuracy associated with the WGS 84 datum ensemble. Map layers have been topologically aligned with each other, creating an illusion of accuracy. However, all layers have all been equally misaligned. This occurs because:

• National mapping agencies store maps accurately in their region’s official static datum.
• Until recently, a region’s official datum only had one transformation to the WGS 84 datum.
• As a time-dependent transformation was not applied, this locked the target datum to a point in time, effectively emulating a static datum.
• For instance, the transformation from CRS GDA94 (EPSG::4283) to CRS WGS 84 (EPSG::4326), (Transform: EPSG::1150) locks transformed coordinates to the year 1994.0.

Problem 4.1: Coordinates systematically misaligned:

Maps sourced into the WGS 84 datum ensemble are regularly systematically misaligned.

Problem 4.2: Lack of awareness:

The poor accuracy of WGS 84 has been masked from users as they see topologically aligned maps. Effectively, there has been a communication gap between the geospatial community and software developers in understanding this problem.

Problem 4.3: WGS 84 being used as if it were an accurate static datum

In web-mapping, WGS 84 is regularly implemented as an accurate static datum. Datasets are consistently transformed from ‘ground truthed’ maps in national/regional datums via null transformation to WGS 84, resulting in consistently misaligned maps, which create the illusion of accuracy.

Problem 4.4: Misalignment with GPS sourced points

WGS 84 features sourced from current GPS devices (such as mobile phones), will be misaligned with WGS 84 maps transformed from a region’s official static datum. The tectonic plate coordinate shift will be exposed.

Problem 4.5: Different epochs in each region:

WGS 84 uses null transformations to datums at different epochs in different regions. In practice, this means WGS 84 is aligned with different epochs (dates) in different regions. For example:

• Australia’s epoch of WGS 84 is aligned with GDA94 (EPSG::4283), which is aligned to ITRF92 at epoch 1994.0 (Transform EPSG::1150).
• USA’s epoch of WGS 84 is aligned to CRS NAD83 (EPSG::4269) from 1986.0 (Transform EPSG::1188) etc.

This will make it difficult for the world to agree to lock in a previously used WGS 84 epoch for web-mapping.

5. Australia’s misaligned web-maps

Australia had defined our static GDA94 datum as coincident to ITRF1992 on 1994-01-01 which was coincident to WGS 84 (G730) at that time, leading to defining a null transformation between GDA94 and WGS 84 (EPSG::1150).

However, due to Australia’s fast-moving tectonic plate (~7cm per year), coordinates in the dynamic WGS 84 datum drift apart from static GDA94 coordinates. In 25 years, this equates to ~ 1.8 metres.

Modern GPS positioning, which uses WGS 84 coordinates, and is projected to have centimetre level accuracy within the next few years, now have metre level misalignment with GDA94 sourced basemaps.

We have “mountain to Mohammed or Mohammed to mountain” options to address this misalignment:

1. Move basemaps and datum to align with current GPS (WGS 84) positions. (Move mountain to Mohammed).
2. Adjust coordinates in GPS mobile devices to account for WGS 84 and GDA94 differences. (Mohammed goes to the mountain).

Australia chose option 1, to gazette a new official national datum, GDA2020, which will be coincident with WGS 84 (G1762) and ITRF2014 on 2020-01-01.

Figure: Misalignment of WGS 84 Web Mecator web-maps
when sourced from GDA94 and GDA2020

The following datum transformations have been defined:

1. GDA94 - GDA2020 (EPSG::8048) accounts for Australia’s drift of ~ 1.8 metres.
2. GDA94 - WGS 84 (EPSG::1150), a null transformation which assumes GDA94 is the same as WGS 84.
3. GDA2020 - WGS 84 (EPSG::8450) a null transformation which assumes GDA2020 is the same as WGS 84.

Problem 5.1: Systematic misaligned maps

Combining WGS 84 map layers, sourced from GDA94 and GDA2020 via null transformations, results in the systematic misaligned by ~ 1.8 metres. (Previously these layers would have been aligned because they would have both been transformed from GDA94.)

Problem 5.2: Reverting back to GDA94 in web-mapping

To facilitate backward compatibility with existing WGS 84 based web-mapping services, Australia plan to continue aligning new WGS 84 based web-maps with the old GDA94 instead of with the new GDA2020 (as originally planned).

Problem 5.3: Equating a static and dynamic datum

Defining a static and dynamic datum as equivalent will inevitably lead to inaccuracies which will increase over time, and will need to be addressed in future.

Problem 5.4 Moving datums is costly

Moving to a new national/regional datum to improve accuracy requires significant effort, and in Australia’s case, we discovered that the widespread adoption of web-mapping introduced extra hurdles.

Notably, new coordinate operations, which support moving coordinates between epochs, can be implemented to address the same accuracy business drivers.

These options should be considered by those considering future updates to national/regional datums.

Problem 5.5 GDA2020 - WGS 84 transformation not implemented:

Some software providers (such as ArcGIS server), which is used by many Australian mapping agencies, have not implemented the GDA2020 - WGS 84 (EPSG::8450) transformation, restricting these agencies from supporting it. (Considering our web-mapping misalignment problem, this is probably a good thing.)

6. Reference frames and projections

Problem 6.1: WGS 84 being used instead of modern ITRF

The WGS 84 Reference Frame used in web mapping is not as accurate as the International Terrestrial Reference Frame (ITRF). Compared to WGS 84, ITRF is:

• The baseline reference frame used in geodesy
• The reference frame being adopted by national datum modernisation initiatives
• Addresses time
• Has more accurate transformations into it.

Future global datum usage should consider moving from WGS 84 to ITRF.

Note 6.2: Spherical instead of Ellipsoidal Datum:

The WGS 84 Web Mercator projection used in web-mapping is based on a sphere, not an ellipsoid. This results in coordinates being distorted by up to tens of kilometers. (Distortions increase the closer to the poles). Further, the projection is non-conformal, which means the amount of distortion in the x axis differs to the y axis. Consequently derived distances, areas, bearings and so on will also be distorted.

If the world moves from WGS 84 to another CRS for web-mapping, along with the global retiling of maps entailed, this would be an opportunity to reconsider moving to an ellipsoidal based projection.

7 Constructs to describe accuracy

• Transforming coordinates between datums has associated inaccuracies, which differs depending on the transformation path taken, and the transformation path typically is not predetermined.
• In contrast, converting coordinates from one reference system to another, (e.g. from latitude /longitude to easting /northing) is a mathematical procedure which does not introduce inaccuracy.
• The lineage of how a dataset’s coordinates are derived is typically not recorded with the dataset.
• By definition, transformations between datums are assigned an accuracy, but accuracy isn’t assigned to a datum per se.
• Dataset formats and web service standards typically don’t record or publish accuracy statements.

Problem 7.1 Difficult to determine dataset accuracy

As standard spatial data formats and web service interfaces don’t define attributes for accuracy, it is difficult for users to determine the accuracy of a datasets. Equally, it is difficult to publish a dataset’s accuracy.

Problem 7.2 Inaccuracy introduced during round-trip transformations

There are multiple transformation paths between datums. For instance, transforming between a GDA94 source and GDA2020 target datum can apply:

• EPSG::8048, Helmert 7 Parameter transformation (1cm accuracy)
• EPSG::8446, NTv2 Conformal transformation (5cm accuracy)
• EPSG::8447, NTv2 Conformal and Distortion transformation (5cm accuracy)

Further, transforms can be applied directly (eg: GDA94 -> GDA2020) or via a hub transformation (eg: GDA94 -> WGS 84 hub -> GDA2020). Direct or hub potentially provides different results with different accuracies.

Because transformation lineage is typically not recorded with a dataset, transforming back to a source datum can introduce inaccuracy due to a different reverse transformation being applied.

Note: For proj based implementations: the proj6 library will use the most accurate transformation, if available on the system, otherwise it will fallback to less accurate transformation available, and provide hints on where to download more accurate transformations.

Problem 7.3 Describing accuracy

We don’t have the means to convey to a data administrator that a datum includes inaccurate transformations into it (and hence should be avoided for accurate use cases) - E.g. for the WGS 84 datum ensemble.

Problem 7.4 Lack of awareness of accuracy

There appears to be a general lack of discussion and awareness about these accuracy challenges.

8. Communicating map concepts

Discussion about mapping is hindered by:

• A complicated technical problem,
• Being applied to solve multiple use cases, with differing requirements and a variety of implemented solutions.
• Multiple user groups understanding different facets of the problem, often using differing terminology, and talking past each other.
• Historical implementations sometimes clouding people’s understanding.
• Message crafting typically being dominated by highly technical people with minimal access to technical writers, professional educators and communicators.

Problem 8.1 Confusing terminology

Understanding the intricacies of mapping is complicated by terminology which is often misused, overloaded or misunderstood.

Problem 8.2 Confused messaging

Based on questions and misunderstandings seen on email lists, there appears to be a lack of clear messaging and education around projections, datums, coordinate reference systems, transformations, EPSG codes and related concepts. Messaging would benefit from input by communication and education expertise, tailored for key user groups.

Problem 8.3 Abstract out complexity

We should simplify developer and user experiences by abstracting complicated concepts behind simple implementations.

Web-mapping terminology (simplified)

Understanding the intricacies of mapping is complicated by terminology which is often misused, overloaded or misunderstood.

Hopefully, these simplified explanations should help. For official definitions, refer to the OGC Abstract Specification Topic 2: Referencing by coordinates.

• Datum (more recently called a Reference Frame): A reference frame is a mathematical model of the earth against which features can be represented as coordinates.
• Within a static datum, a feature’s coordinates are locked in time, or locked to its tectonic plate, so the feature’s coordinates remain the same over time. For example, GDA2020 is a static datum for Australia.
• Within a dynamic datum, coordinates are fixed to the earth as a whole. As the earth’s tectonic plates move (by a few centimeters a year), a feature’s coordinates also move. Examples include WGS 84 as currently used by Global Positioning Systems (GPS), and the current realization of the International Terrestrial Reference Frame (ITRF2014).
• Datum Ensemble: A group of related realizations of a Reference Frame (datum) used for lower accuracy applications where differences are insignificant. The WGS 84 datum (EPSG::6326) used in web-mapping is a datum ensemble.
• Coordinate System: A set of mathematical rules which specify how coordinates are assigned to points.
• Coordinate Reference System (CRS): A coordinate system that is related to the real world by a datum. A CRS is typically referenced in Web Service calls by an EPSG code. EPSG::4326 is the defacto standard CRS used in web-mapping and refers to the WGS 84 datum ensemble (EPSG::6326).
• Epoch: A point in time, as applied to time dependent datums, expressed in decimal years. For example, 2017-03-25 is epoch 2017.23.

Multiple definitions of epoch are commonly used to describe different aspects of a coordinate and a CRS, which can lead to confusion:

1. Observation epoch: The date at which the observation was made.
2. Coordinate epoch: The date at which the coordinates are expressed.
3. Realization epoch: Part of the Indicates in general the date at which a datum was defined.
4. Reference epoch: The epoch at which time-dependent coordinate transformation parameters are defined, along with their rates of change. (The EPSG database currently calls this “Realization epoch”.)

• Map projection: Coordinate conversion from the earth’s ellipsoidal coordinate system to a flat plane. WGS 84 Web Mercator (EPSG:3857) is the defacto standard projection used in web-mapping. (Its official name is “Pseudo Mercator”). WGS 84 Web Mercator is projected from the WG84 datum ensemble (EPSG::6326).
• EPSG Codes: Online database that contains definitions of numerous datums and map projections, along with formulas to translate between them. Each is uniquely identified via an EPSG code.
• Coordinate operations:
• Coordinate conversion: Changes coordinates from one coordinate reference system to another coordinate reference system based on the same datum (reference frame).
• Coordinate transformation: Changes coordinates from one coordinate reference system to another coordinate reference system which is based on another datum (reference frame).
• Point Motion Operation: Changes coordinates within one coordinate reference system to account for the motion of the point within the CRS over a period of time.