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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

WestCorkPalaeo
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    How did you get this geotiff exactly? – Kasper May 23 '23 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 https://i.stack.imgur.com/oiHBo.png) – user2856 May 24 '23 at 02:16
  • 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: https://i.stack.imgur.com/Q6z4P.png Than play around with the settings to get a feeling – Babel May 24 '23 at 09:49
  • 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 '23 at 09:52
  • OK, all newly posted screenshots look OK, except the last one. Unfortunately, you skipped exactly this last step: how do get from the second last to the last screenshot, what happens in between? How do you export, what are your settings? 3rd last and 2nd last screenshot seem to be the same, so there seems to be missing something that is crucial to understand what goes wrong? – Babel May 24 '23 at 19:42
  • Can you share the saved bathymetry raster? – Babel May 24 '23 at 19:44
  • It seems to me the problem arises before I even try saving the Mapzen as a separate raster layer. I use hillshade so that I can see the map - surely I should have something visible without needing to do that? If I change the colour ramp then I get a single colour - corresponding to the highest value. So that indicates to me that the values are already high, before I have even done anything. Notice how in the penultimate screenshot above the map has changed to deep red - corresponding to a value of 0, or higher. For SW Ireland I would expect to see lower values and thus other colours. – WestCorkPalaeo May 25 '23 at 09:36
  • Please, read the comments carefully and proceed as described. Otherwise, it's impossible to help. 1) Having a "single color" does not mean anything: again, if you have a raster from -10.000 to +9.000, an extract with range from -100 to +200 results in a +/- monochromatic image. What is really important is the min/max raster values like in this image: https://i.stack.imgur.com/TWl9O.png - on your screenshots, those values are hidden. Show us these values! – Babel May 25 '23 at 10:24
  • Three different users were asking above to describe in detail how you save the raster, but you don't tell us and you don't include screenshots showing your settings when saving. But this is crucial! Create a new project, load Mapzen as layer, don't touch it, only right click > export it and describe us in detail what settings you make and include screenshots of the export dialog. Then share the resulting raster file with us for inspection.
    • – Babel May 25 '23 at 10:25
  • I save the raster by right clicking on mapzen in the layers panel - Export - Save As - Geotiff. See later screenshots. – WestCorkPalaeo May 25 '23 at 13:01
  • How do I share the tif file? It's 37 kB. – WestCorkPalaeo May 26 '23 at 05:35
  • Upload it to a cloud and share the link here – Babel May 26 '23 at 05:50
  • If you refer to this image: https://i.stack.imgur.com/wP8a9.jpg - here, we can't see the raster's min/max as you manually set min/max for styling. Please read again what I wrote yesterday: show us the original layer, without any manual changes that only add confusion. 2) If you refer to this imagage: https://i.stack.imgur.com/VpdxC.jpg you have checket Create VRT. Uncheck this and save Raw data Tiff. 3) At the end of your question, you added an empty (completely white) png image instead of a tiff file.
    • – Babel May 26 '23 at 07:21
  • It is quite difficult to see what happens when you provide only incomplete or confusing information that hides the essential parts. I once again advice you to read carefully the comments and proceed as described there. Up to know, I can't see what goes on and suspect that it might be a bug. Have to tried to do that with a newer QGIS version to see if the problem persists? – Babel May 26 '23 at 07:24
  • I tested it with QGIS 3.24.0 and it works without problem. So there must be some additional step you do. I ask you once again to proceed step by step and do nothing else then 1) loading Mapzen to Layers panel, 2) right click > export, 3) save as Raw data GeoTIFF. See here and proceed exactly as shown on these screenshot and don't do anything else than what you see there and show us the exactly same screenshots: https://i.stack.imgur.com/ktvxz.png – Babel May 26 '23 at 07:54
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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? – WestCorkPalaeo May 26 '23 at 10:15
  • No idea. In 3.24.0, it worked well for me, not sure if a bug was added for pointrelease 3.24.2. In the 3.24.2 you could try to create a new, clean user profile and see if the problem persists. Howver, you solved your problem. So maybe you want to add this as an answer so that people with similar problems see what helped in your case. – Babel May 26 '23 at 11:58
  • Well, anyway, thank you for all your help and time and effort. You were a great help. I appreciate it. – WestCorkPalaeo May 26 '23 at 12:46