Played with Potlatch "revert" function. Bit painful since you can't step quickly through versions or see easily what has changed between versions (e.g. attribute change or spatial change, or spatial but not in the area you're zoomed in on).

Spent more time assessing accuracy. I have been using Global Mapper (GM) v10.02 which can load and display .osm, .shp, .sid. I have overlaid OSM / NRCan (Geobase / Canvec / whatever), SNB's 1:10k topo, SNB's SID orthophotos. Findings, in no particular order:

- the SIDs are quite useful, very useful since a lot of the Yahoo imagery has too coarse a resolution to resolve roads. From what I recall of the SID technical documentation, the accuracy is supposed to be fairly good, well it says "+/- 6.0 metres for well-defined features" here:
http://www.snb.ca/gdam-igec/e/2900e_1b_i.asp

More tech. information here: http://www.snb.ca/topo/assistance/ORE1999R.pdf. It says 90% of well-defined features (i.e. not covered by vegetation) must fall within 4m of their true position. Whenever I've plotted GPS positions on these images I have been comfortable with the locations. I think I am going to have faith in the accuracy of the SODB (Soft Copy Orthophoto Database), as an underlay to vector data to support assessment or even for on-screen digitizing.

Currency: imagery dates from 1996 - 2002. Obviously some roads have changed since then but most have not.

Legality of using SNB's data: see e.g. http://www.snb.ca/gdam-igec/e/2900e_1b_v_.asp?OrthoNum=45756660. From my reading, it says: "do what you like with it but don't blame us if you have problems".

- the SNB Digital Topographic Database (DTDB) has data in SHP format. This is topo data at a nominal 1:10,000 scale. Excellent stuff but perhaps more detailed than necessary. There are 1894 map sheets.

- the NRCan Geobase has data in SHP format on NAD83. I am interested in roads, especially their accuracy. I am also interested in grabbing water, forest, swamp, to supply context.

A thought I am having: it may be best in general to use Geobase roads because the Geobase data all fits together well. Bad to use someone's hokey GPS trace for a road and find it crossing the edge of a Geobase forest or lake.

The Geobase roads (actually the NRN roads) seem to fit the SODB pretty well. A problem with the NRN roads is that they are too filtered, especially at curves: sometimes curves become too jagged and the straight segments get noticeably far from the true curved roads.

The NRN data for NB is relatively old (2003) and does not have a great deal of attribution. But it is pretty complete so very attractive for using.

Merging with existing OSM roads: fortunately there is not a huge amount of roads in OSM already, for NB. This makes merging less of a problem. On the one hand I am inclined to replace existing OSM roads with NRN ones, for consistency and possible accuracy. On the other hand it does not seem right to go knocking out other people's possibly good work just because "it wasn't invented here".

For roads, it makes sense to do the main highways first starting with the TCH. I need to find out what the preferred attribution is supposed to be for Canada.

Water: I am attracted to the idea of doing rivers and lakes first as these do not interfere with anyone else's work. Again I need to find out about the preferred attribution.

Some other attributes: we should really record source, accuracy, source date.

Tracy: looked at GPS trace + OSM road on rte 101 through Tracy. Hmm, GPS trace seems about right but OSM road is about 70m too far E.

How can I insert images into the diary ??

Oh yes - Yahoo imagery seems a bit off sometimes in Fredericton, a few metres, nothing drastic.

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