FDO RFC 60 - Add OGC Annotation Support to the FDO API

This page contains a request for comments document (RFC) for the FDO Open Source project. More FDO RFCs can be found on the RFCs page.

Status

RFC Template Version	1.1
Submission Date	April 13, 2011
Last Modified	Greg Boone, April 13, 2011
Author	Greg Boone
RFC Status	Not Ready For Review
Implementation Status	Not Started
Proposed Milestone	3.7.0.0
Assigned PSC guide(s)	Greg Boone
Voting History	(vote date)
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Overview

The RFC is being proposed in order to add formal OGC Annotation Support to the FDO API.

Motivation

Spatially placed text is a common requirement of applications. Many application have stored their text placement information in proprietary manners due lack of a consistent and usable standard. Although the mechanisms for text storage have tended to be compatible, the actual format for exchange has been sufficiently different, and, therefore, non-standardized to interfere with complete data exchange and common usage.

Annotation text is simply placed text that can carry either geographically-related or ad-hoc data and process-related information is displayable text. This text may be used for display in editors or in simpler maps. It is usually lacking in full cartographic quality, but may act as an approximation to such text as needed by any application.

The primary purpose of standardizing this concept is to enable any application using any version of Simple Features data storage or XML to read and write text objects that will describe where and how the text should be displayed. This design ensures that applications that do text placement should have no problem storing their results and that applications that comply with the standard should have no problem exchanging information on text and its placement. Unlike spatial geometries, text display is very dependent on client text rendering engines and the style and layout attributes applied. The spatial area covered by text is only partially determined by the locating geometry. Style and layout attributes along with the actual text and locating geometry are all needed to display text correctly. Thus, it is critical to have a place to store these attributes in the feature database. While it is impossible to guarantee absolute fidelity of display on all rendering systems, applications can interoperate at a useful level.

The most common perception of text display is for cartographic purposes, for printed maps of high technical and artistic quality. While this is a potential use of placed text, its more every-day use is for identification of features in any display, regardless of the purpose of that display. So both cartographic preprint and data collection edit displays have a requirement for placed-text, albeit at different levels of artistic quality. The purpose is still the same, to aid in the understanding of the “mapped” features, either for map use or feature edit and analysis.

Text can also be used for less precise annotation purposes and more for quick display of text labels that make a display more understandable. The text so placed may not even have any associations to real-world features, but may be used to store information pertinent to the process that the data is undergoing at the moment. Thus, in a data collecting and edit display, a particular placed text may be used to indicate an error in the data that needs to be resolved, such as “sliver,” “gap” and “loop” error in digitization. Here the annotation is placed near the geometric error, but is not necessarily associated to a particular feature, as much as to a portion or portions of feature geometry objects.

Annotation text can include text on maps derived from vector information, or text overlays for imagery for information not discernable from the image, such as place or street names. In most cases, applications that do this have certain rules for creating and re-creating text based on the dynamic view of the mapping application. While this standard is not targeted to those usages, there are some allowances for this type of storage if it is so desired. In particular, it is allowable to store text that does not scale with the map objects but instead has a fixed display size (expressed as “points”, 72 to the inch). However, there are some limitations on this usage particularly with spatial indexing.

Proposed Solution

TBD

Implications

TBD

Test Plan

TBD

Funding/Resources

Autodesk to provide resources / funding