Cemetery Slopes

Question

I am working on a new cemetery GIS.  Our sexton has given me the dimensions of grave sites in the various sections of the cemeteries.  All of the plots are drawn in.  However, these measured polygons do not line up with our aerial photography because of the slopes.  A 10-foot measurement on the slope using a tape measure can equate to something like 9.8 feet when looked at from above.  This slight difference propagates itself downhill, as headstones appears to be offset further and further from their grave polygons.  Can you all give me some ideas on what to do to line up the plot polygons with the imagery?  Feel free to ask any clarifying questions.

jcarlson · Answer

Yes, my avatar is a GIF! And lovingly hand-crafted, I might add.

So, if you've got a good DEM and access to the imagery itself, I would definitely rubbersheet the imagery to the DEM. It may appear somewhat distorted when viewed from above, but distorted and accurately placed is better than being offset.

RogerDunnGIS · Answer

I like your Tetris avatar!  Reminds me of the good old days of high school on a Gameboy.  I just want to click it and turn the shapes.  Is your avatar a gif?

The imagery is ours, and I'm pretty sure we have a lot of good 3D data somewhere on our network.  I think we even have it in various formats like DEM and TIN.  Anything else I should look for?

Since the plots, lots, blocks, sections, and cemetery boundaries are all drawn up and attributed, what would be the best way to use my 3D data to correct what I have already done?  Should I rubbersheet our imagery over a DEM or TIN and try to line up the plots with headstones that way?  Any hints in this area would be helpful.

One thing I've thought of is to have one of our crews GPS a lot of key points in the cemetery which translate to features I already have.  Then maybe I can use the transformation tools to kern the cemetery correctly.  But then I worry if I'll end up with rectangles (desired) or shapes that look more like rhombuses (not desired).  But I don't want to send someone out in the field to redundant data that I can find from the comfort of my office.

Any other hints or advice you and the community could provide would go a long way.  Thank you.

jcarlson · Answer

Hello, Roger! It's quite a tricky problem. Do you have access to the imagery files, or are you just using a basemap from elsewhere? And do you have access to good elevation data for your area?

One way to do this would be to work in a 3D environment with Z values on your vertices. You can create a line that follows the elevation surface, pulling its Z values from it. Such a line could measure 10 feet in length, but then when the same feature is viewed in a 2D map, ought to line up with your imagery.

I had to digitize features for a university campus once, and the changes in elevation made it pretty tricky. But it was made much easier with access to a good DEM.