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I wish to obtain bathymetric data (depth) for the offshore region of SW Ireland. One source is Mapzen. I can connect fine, but the elevation values in each cell are crazy - multiband color gives min 806533 to max 836172; singleband grey min 797774 to max 845153; and singleband pseudocolor min 797774.375 to max 845152.8125.

I just want to create bathymetric contours, but I can't with these.

I have a similar problem with the EMODnet TIFF file.

EDIT

This actually arose out of trying a solution to this problem

Filling gaps in vector contour line

What is happening and how can I get 'real' values? Please. I am using QGIS 3.24.2 Tisler.

enter image description here

These are the steps to create the layer - Open new project In 'Browser' panel, select XYZ Tiles / Mapzen Global Terrain and drag into 'Layers' panel. Although the band indicates max value 9,000 and min value 10,000, all that displays is either a white box (or black if 'white to black' i s selected as the colour gradient), or deep red if singleband pseudocolour is selected - i.e. the colour that corresponds to the highest value. This value is supposedly 9000, but clearly there are no lower values, and in fact all values are MUCH higher. Selecting 'hillshade' gives a good representation, and contours sort of works if, after zooming in to my area of interest, I set contour interval to a high number, - 1000.

I showed min/max values for multiband just for completeness of all the options on that layer's symbology, even though it is, as you say, a singleband.

Text from the layer 'Information' tab below

    Name    Bathymetry
Path    C:\Users\user\*****\Bathymetry.vrt
Size    4 KB
Last modified   23 May 2023 13:44:50
Provider    gdal


Information from provider
Extent  -1284154.7424000001046807,6597507.2053000004962087 : -846374.8812000000616536,6913681.5931000001728535
Width   4378
Height  3162
Data type   Float32 - Thirty two bit floating point
GDAL Driver Description VRT
GDAL Driver Metadata    Virtual Raster
Dataset Description C:/Users/user/****/Bathymetry.vrt
Compression 
Band 1  
STATISTICS_APPROXIMATE=YES
STATISTICS_MAXIMUM=845152.8125
STATISTICS_MEAN=827315.16254624
STATISTICS_MINIMUM=797774.375
STATISTICS_STDDEV=4858.0862917786
STATISTICS_VALID_PERCENT=100
More information    
Dimensions  X: 4378 Y: 3162 Bands: 1
Origin  -1.28415e+06,6.91368e+06
Pixel Size  99.99540000000000362,-99.99190000000000111

Coordinate Reference System (CRS)
Name    EPSG:3857 - WGS 84 / Pseudo-Mercator
Units   meters
Method  Mercator
Celestial body  Earth
Accuracy    Based on World Geodetic System 1984 ensemble (EPSG:6326), which has a limited accuracy of at best 2 meters.
Reference   Dynamic (relies on a datum which is not plate-fixed)

Identification
Identifier  
Parent Identifier   
Title   
Type    
Language    
Abstract    
Categories  
Keywords    

Extent
CRS 
Spatial Extent  CRS: - Projected
X Minimum: 0
Y Minimum: 0
X Maximum: 0
Y Maximum: 0
Temporal Extent Start: 
End:

Access
Fees    
Licenses    
Rights  
Constraints 

Bands
Band count  1

Number  Band    No-Data Min Max
1   Band 1  n/a 797774.3750000000   845152.8125000000

OK, so here are a series of screenshots starting from New Project....

New Project Select Mapzen Layer Properties Adjust min and max values Change to Hillshade (can't see anything otherwise) Hillsahde displayed Now I can see it, I can zoom in on SW Ireland

Back to Layer Properties Change to singleband pseudocolor So now the map goes to max colour value - not good! Export layer and there are the big max and min values

Later Screenshots of the Export process to save as a layer

Start new project next - properties right click on mapzen layer - export - save as and that is the rest of the save as box showing all other options

But that didn't work because I hadn't used hillshade to zoom in on SW Ireland - and the processes to download the data, all global data, timed out. We live in SW Ireland - the worst internet connectivity in Western Europe. So maybe that is the issue. But even if I specify a small area, a tiny area, and the download process completes, I still get the crazy values.

I have also added three more screenshots. I dragged mapzen from the browser panel to the layers panel, changed to hillshade so I could see something, focussed in on a very small area between Whiddy Island and the southern shore of Bantry Bay where I know water depth is no more than 20m and land elevation no more than 100m. Change to pseudocolour etc. and.... deep red - indicating that ALL elevations are greater than 100. So I change the range max to 100,000 - still deep red - indicating the elevations are greater than 100,000. And that is nothing to do with saving the layer. So something is going awry with the incoming data, or the settings I have for importing / downloading data

1 2 3

Here are screenshots of my exporting - save as tiff file of a small extract - Whiddy Island.

1 2

And here is the tiff file

3

OK so. 1 2

18
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    How did you get this geotiff exactly?
    – Kasper
    May 23, 2023 at 17:40
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    Yes, please edit your question to specify exactly how you got that data as you shouldn't have multiband data (did you export a rendered image by accident?) and also include a copy (select the text and copy it, don't screenshot it) of the text in the QGIS Layer Properties -> Information tab. I just extracted some data from the Mapzen Global Terrain tiles in QGIS and it has sensible values (example i.stack.imgur.com/oiHBo.png)
    – user2856
    May 24, 2023 at 2:16
  • Drag Mapzen to layers panel. You have a range of -11,000 to 9,000, but you don't see any difference in the color. That's normal as around Ireland, you just have a small range of maybe -1000 to 800, so compared to the whole range, that is just a small spectre. manually set the min/max limits to e.g. -800 to 0 and set mode to equal interval > 12 classes, see: i.stack.imgur.com/Q6z4P.png Than play around with the settings to get a feeling
    – Babel
    May 24, 2023 at 9:49
  • I still don't understand what you did to get such large min/max values. At the start, you had -11,000 to 9,000 when you drag and drop Mapzep to layers panel. Can you describe step by step with screenshot what you did to get min 806533 to max 836172? This is not normal, min/max should stay the same whatever layer rendering style you use. Don't crop the screenshots, show us your whole QGIS window and do not make any changes that you don't describe with text.
    – Babel
    May 24, 2023 at 9:52
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    I have installed 3.30.2 and I can download and save the data no problem. So as you suggested, it looks like a bug for that version? Maybe? May 26, 2023 at 10:15

1 Answer 1

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It would appear that the solution to this problem lay with the version of QGIS. Possibly something not right with the process that accesses the Mapzen data.This is with reference to QGIS 3.24.2. The process works fine with 3.24.0 and also with 3.30.2.

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