How accurate is GIS mapping ?

[intexasatthe moment]

I have no idea what GIS is ,but currently involved in discussion with Southwark planning about disputed width of access road .

Going on for some years ,relates to access to backland .

Planning dept always refer this particular point over to Highways ,I always ask for site visit and measurement ,but this never seems to happen .

Various figures have been quoted by council representatives ,architects et al .

Latest response is from Southwark quoting reference to GIS mapping .

Does anyone know if this is equivalent of visiting site and measuring ?

Sorry if I'm being dense here .

[Huguenot]

Not being dense at all.

People often confuse GIS with satellite location systems, remember that these have quite wide margins of error and then wonder about their accuracy.

However GIS is a more of a catch-all term for all the different ways the geography of an area can be measured.

In that sense, when a worker rocks up and measures the road with a tape they're actually contributing to the GIS. They're not two different things.

For the most part, modern GIS systems are populated using photographs. Certain distances on the photograph will be measured very accurately, and then the information for surrounding areas calculated from comparing the locations in the photograph.

In general it's regarded as very accurate.

[intexasatthe moment]

Thanks .

Well given Southwark's reluctance to visit and use a tape measure ,I'm guessing that this GIS will be dependent on photographs ,or ( head hurting here ) working back from a marker that designates a point that someone did actually measure ?

So do you think that modern technology can give a measurement from a photograph which is accurate to within 0.1 of a metre ,or as I tend to think of it ,nearly 4 inches ?

( no comments please about the contradictory nature of my desire for accuracy and my use of the term "nearly ".?

[ianr]

It might be worth having a look at the online GIS-incorporated Southwark maps, linked to from here.

[intexasatthe moment]

Thanks ianr - then I guess I measure and scale it off the map ?

But will the data it's based on be valid in the first case ?( photos I guess ok ,but ....what else ,extrapolations from other distant points ? )

I just can't quite get my head ( aged and neither numerate nor technlogically up to date ) round depending on something that feels remote and dependent on variables ( photos v.other measurements ... )as producing results that are accurate to within the odd half ,and less ,metre .( a unit which I'm not even sure I know how to spell .)

[Chener Books]

The Ordnance Survey technical standard for "accuracyofposition" data is:

"1.0m Urban data capture standards (1:1250 scale). This is the nominal accuracy of a point position at the 99% confidence level."

This means a 5 metre line measured by tape on the ground could be between 3 metres and 7 metres when scaled off a map.

Before Southwark Council makes assertions about the accuracy of its GIS mapping it needs to specify:

1) the accuracy of the data set input to the GIS software

2) the paramaters used in manipulating the data set within the GIS software

3) the accuracy of the output presentation from the GIS software.

John K

[intexasatthe moment]

Thank you John - that's really helpful .