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flyash8 post(s)
#07-Apr-16 10:16

We have purchased NEXTMAP DSM and DTM 5m posting with 1m vertcal RMSE.

When I load the surface into Manifold it shows 1 elevation value per pixel whereas if I load the same surface to global mapper, it virtually changes elevation within the 5m pixel. It seems Manifold is just getting average elevation per pixel.

The surface is in Geotiff format.

Can you suggest ways for me to get the elevation values as it seems incapable to show the values being shown by global mapper?

Thanks in advance for your help guys!

tjhb
10,094 post(s)
#07-Apr-16 10:56

You have set Global Mapper to interpolate elevation values smoothly between elevation centres, reporting interpolated Z values in the status bar. It does this using bilinear interpolation by default for elevation values, but there are several other options, including no interpolation (nearest neighbour).

It's arguable that this is appropriate for rasters modelled as sample points (grid-centred) though not for grids modelled as squares or pixels (cell-centred). There's room for disagreement on that though, and there is no single right answer for all data on either paradigm or for all purposes.

Manifold does interpolate automatically for some purposes (including Surface Transform and reprojection) but not for Z readout in the status bar. (This is broadly in line with Manifold treating all surfaces as cell-centred, not grid-centred--but again there's room for disagreement.)

Note that Manifold is not reporting averages--on the contrary it is reporting actual data values, without blending or smoothing.

Why is it important to you that you get interpolated elevation values reported in the status bar, for this data?

flyash8 post(s)
#07-Apr-16 11:32

Hi tjhb,

Thanks for the detailed answer! All along I thought it was Manifold issue. And that makes perfect sense as the raw data from nextmap has 5m posting.

I use Manifold to generate my pipe node elevation (via transfer heights) for hydraulic modeling. And it gets very tricky during calibration of my model especially when pipes are located on a ridge.

But in this case Manifold only reflects the actual values as you have mentioned, whereas global mapper does this by interpolation.

Thanks for this information - and yes it does not make sense to interpolated data.

Again many thanks.

Regards,

Ariel D. from the Philippines

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