How can I take numerous shapefiles and have them line up in GIS?

Question

I am trying to take a shapefile of subdivisions within a county that I have created and line it up with another shapefile that was given to me by the County Appraisal District (parcel data). When I try to get them to line up then my streets shapefiles is not aligned with everything else. They are all on the same coordinate system and I do not want to have to recreate the shapefile for the subdivisions. Any thoughts?

Answer 1

Your problem is probably one of projection

| projection: character string that names a map projection to use. See | 'mapproject' (in the 'mapproj' library). The default is to | use a rectangular projection with the aspect ratio chosen so | that longitude and latitude scales are equivalent at the | center of the picture. `-----

Answer 2

Agree that your problem is projection. Is there a .prj file extension associated with either of your files? If not key first is finding out what projects you have. I would guess State Plane of some sort if you are dealing with U.S. centric local data from a local government.

Answer 3

Cadastral tools (surveyer tools) usually let you specify a handful of control points and will then "warp" the data to fit to your control points. This can be anything from a simple shift to something more complex. If everything is shifted by a few feet, you can also just use your "editor" to select all shapefiles and then move them however many feet necessary.

If you've verified that both shapefiles are using the same coordinate system, then projection is less likely to be the problem. It's fairly common for parcel data to be "offset" from other data sources (such as roads). This comes from inconsistent collection methods and points of reference.

Another source of error can be that one of the shapefiles has the wrong coordinate system specified. For example, if the roads were actually WGS 1984, but it's prj is set to NAD1983, you will see some significant errors. This usually happens if you had to manually set the coodinate system for a shapefile (i.e. it didn't include a PRJ and you created one).

Answer 4

This is a question with answers that may be simple or may be very complex, depending on your situation. As a GIS developer, I've most commonly seen this as a symptom of an incorrectly defined coordinate system. However, whether this is the case or not, and what the solution is strongly depends on your environment. From here on, I'll assume that you're working in an ESRI package...

I agree with the other posters that your problem is one of mismatching projections and/or datum definitions.

The most important thing to understand as regards projections in ESRI software is this:

Manually setting the projection of a dataset (shapefile, geodatabase feature class, etc) in ArcCatalog does NOT reproject that dataset!!!

In order to reproject your data, you must EXPORT the data from an ArcMap session in which you've been working and where the data is obviously lined up correctly. During the EXPORT, you are given the choice of saving your data with the coordinate system of the underlying map or that of the original dataset.

Your best bet is to follow these steps to create a new dataset with the correct projection and then extrapolate what you need to do to fix your specific problem:

1. Create a new ArcMap session and set its coordinate system:

Do this in a fresh ArcMap session with NO OTHER DATA. Be sure to explicitly set the coordinate system of the ArcMap mapview to your desired coordinate system (I recommend the one that matches the data you're trying to overlay, or one from another well-established dataset).

1. Add one other dataset with a known good coordinate system.

2. Create your new dataset in this ArcMap session. Give your new data the same coordinate system as the ArcMap mapview and the one other dataset in the map. Set the XY domain of the new data to exceed the area defined by your other dataset, but don't go beyond the size that will reduce your desired spatial resolution.

3. Create your data. It can be any data at this point. Some lines, some polygons, etc. Save your work.

4. Export your new dataset. When prompted, choose to save with the coordinate system of the underlying mapview.

5. Create a new ArcMap session and add your new dataset. Then add your parcel dataset. They should now occupy the same space in your map window.

6. Edit your new data to your heart's content.

Some probable issues if this doesn't help:

• You didn't follow these steps correctly - check the ESRI documentation; this is a well documented issue.

• The parcel data you're trying to match doesn't have properly defined coordinate system. It's always possible that the keepers of this data don't know what they're doing and have munged it up. I've seen this problem more times that I care to admit.

• You've matched the projection but have mis-matched the datum. Many municipalities are still using data in NAD27, which is way out of date. Some have moved to the modern NAD83. The difference can be up to 300 meters, depending on where in the country you are. Also, data that originates from surveying or GPS equipment is usually collected in WGS84 (the typical default for satellite surveying), which is for all practical purposes the same as NAD83, at least at mapping scale resolutions.

Try researching these issues and see how it goes. I'll say it again:

Manually setting the projection does NOT actually project that data!!

Good luck!