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mmyoung

Shapefile origin

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I'm new to DTMs (which ought to be obvious from my question):

When I import a stack of shapefiles, they align perfectly with respect to one another, but they can be offset from the "paper space" origin (0,0) of Vectorworks by miles.

It's all right; I move my page over to them.

But how is the origin of a shapefile established?

Is there a mapping protocol that determines where (0,0) is? Is there some sort of master grid it references?

Edited by mmyoung

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There are thousands of master grids. Anyone can create one. There are even hundreds of National grids and subgrids, used by the Surveyors General etc.

Now, most GIS-programs use the Long/Lat -coordinate system, which obviously is not relative. When doing conversions, one may apply a Cartesian (often Mercator) projection with a specific origin.

It seems that VW has very limited choices of conversions. Should Albuquerque be in North America, you may be lucky, though.

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The coordinates are the unique longitude and latitude of a point on the earth surface which is based on a particular spheroid used in that region.

The latitude or northing is usually measured from the equator and the longitude or easting from the prime meridian (Greenwich, UK).

Rather than changing the page location use a viewport on a sheet layer for printing. Note that when you create a viewport, it will also be off the page. All you need to do is change the center of the viewport in the object info to 0,0.

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Umm?

I just did a Shape-file export from VW. No long/lat was required. I don't think there is a facility to define one. Or the projection/grid.

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Petri,

When importing a shp file you need to include the projection *.prj file to layout correctly the gis data on a planar surface. VW will save the projection data so if you need to export the data back to a shp file, it will be converted to lat/long coordinates.

Without the prj file or projection data, VW cannot make the conversion as you have experienced.

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Thanks Miguel.

Are there directions somewhere for how to structure the metadata for shapefiles? How to set up .dbf and .prj records? So far I haven't been able to find information I could easily make sense of.

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The shp, dbf, and prj files are created by Arcview. The prj file is a text file that can be edited directly following a specific format but the dbf, which is a dBase 4 database format, would be very difficult to create without the program. Each graphic in the shp file has a unique id which is linked to the dbf (database) that holds the attributes related to each shape.

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Hello, thanks for this info...I'm trying to learn this stuff with my friend who has the gis knoweledge but me...not so much. So it's difficult to toggle back and forth when I don't have those files available for fixing.

Miguel, does this .prj-file work also when trying to get info on excisting dwg map?

We have tried the shapefiles (they come with dpf records not prj)both with AutoCad and with Vector works...and the same thing happens...the shapefiles align nicely with respect to one another,but the map is not where it should be....or rather the map is in the correct location but the shapefiles are off because they seem to have another origin....

Maybe I try to ask the prj-data first and then come back...

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