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Tactics in Lancashire - I've digitised all the SAM, RPG and Conservation Area boundaries from maps of different scales, ages and qualities. First I try and get as modern, clean and clear map as possible from the responsible body - not always easy, e.g. some SAM maps are still only available as county series (i.e. pre-national grid) 1:10,560 extracts that have been copied, written on and copied again and again. Then I compare the supplied maps with the 1:10,000 raster maps and OS Landline mapping on my GIS. Where there is a reasonable correspondence of Landline Arcs with the data on the supplied map I then digitise on screen using the Landline data at 1:500, and follow the appropriate boundaries. Where there is no reasonable correspondence - usually due to redevelopment in the area then you have to make a judgement of which mapping on the GIS to use, or if it is more appropriate to estimate where the lines should fall, or, in extreme cases, scan in the supplied map and rubber-sheet it to fit with your GIS mapping and then digitise over the top of that. I have also on occasion used aerial photographic data on a moorland site to try and make the line drawn around a barrow mean something. Where possible I digitise at 1:500, not to give a spurious accuracy to the data but to make sure that I follow the boundary lines used on the original map. In all cases it is important to record the scale of the original paper map and the scale that the digitising was done at. I also add a date, so that we know when they were last revised. Attribute data is attached to the polygons in the same way that you would add any such data to your GIS, but again acknowledging the source. In any case I always emphasise that this data is *not* the statutory information, but a guide to it and if anyone needs a Barrister-Proof document or map, then they will have to go back to the statutory body, be it Local Authority or EH. I presume that this approach is not unique as other SMRs have added these datasets to their GIS, but thought it might be useful for those who haven't had to do it yet. This isn't an real answer to Nick's question/points, but rather a pragmatic response - i.e. when possible use the most accurate digital dataset but where that's not possible then use the fuzzy raster ***but make sure you document it*** Pete Iles Lancashire SMR -----Original Message----- From: garibaldino [mailto:[log in to unmask]] Sent: 10 October 2001 22:42 To: [log in to unmask] Subject: Re: Where we are. Understanding data and positional accuracy issues a t RCAHMS - Part 2 and Summary Ok as its gone quiet I'll ask a quick question. When digitising SAM's should we be digitising them based on Raster data or digital data? As E Lee pointed out, it is only neccessary to put on both maps to see the disrepancies between the two. And whilst we have argued about the SAM polygons versus the full description being the monument, I am using this example, I think, to highlight a wider point. Given that something is mapped/planned on a more inaccurate base map(ie raster data), should we correct that in the office to match to more accurate digital data, or should we be using the same standard for polygonising (ie stick to the Raster data) until/unless we can re-survey to a more accurate standard whatever the polygon is supposed to represent? As an example, if a polygon of an event covers a field on the raster map, can it be mapped using the digital boundaries of that field and still be correct? Is that even a valid question or have I missed the point here? My gut reaction would be to stick to the more inaccurate standard until the original data can be re-surveyed (if that is possible, which in the case of eg Excavation trenches it may not be), to represent the fuzziness of the data. Any other views? Nick Boldrini ******************** This e-mail contains information intended for the addressee only. It may be confidential and may be the subject of legal and/or professional privilege. If you are not the addressee you are not authorised to disseminate, distribute, copy or use this e-mail or any attachment to it The content may be personal or contain personal opinions and unless specifically stated or followed up in writing, the content cannot be taken to form a contract or to be an expression of the County Council's position. LCC reserves the right to monitor all incoming and outgoing email LCC has taken reasonable steps to ensure that outgoing communications do not contain malicious software and it is your responsibility to carry out any checks on this email before accepting the email and opening attachments. ********************