Issue with samples when I "plot on curve"

[Marge Belcastro]

Hello,

I am trying to wiggle match growth rings to constrain the death of a tree. There are 3 samples that are closely spaced in time (rings 130, 141, and 162) when viewed on “plot dates” things look correct but when I view “plot on curve” I run into an issue where sample 141 is plotting higher than samples 130 and 162 which seem to overlap. 

Here is a bit of the coding used for these three samples:

  Gap( 6);

   R_Date( "NB2019_S162", 1825, 20);

   Gap( 21);

   R_Date( "NB2019_S141", 1795, 20);

   Gap( 11);

   R_Date( "Netarts_NS_TR_130", 1820, 20);

   Gap( 90);

I have verified that all C14 ages entered are correct and I looked at the agreement indices and the lowest agreement is 72% (sample 141) and samples 162 and 168 are both above 75%. 

I am wondering if there is an issue with the curve I am using for calibration (regional vs local).

Below is the OxCal table view of the samples we are working with:

Any suggestions or advice you can give me would be greatly appreciated.

-Marge

[MILLARD, ANDREW R.]

Hello Marge,

 

With your code snippet I get this plot on curve, which looks fine to me. Each date is vertically at its mean radiocarbon date and the probability distributions are the same as in plot dates.  What are you seeing?

 

Best wishes 

Andrew 

-- 

Prof. Andrew Millard 

Department of Archaeology,

Durham University, UK 

Email: A.R.M...@durham.ac.uk  

Personal page: https://www.durham.ac.uk/staff/a-r-millard/ 

Dunbar 1650 MOOC: https://www.futurelearn.com/courses/battle-of-dunbar-1650 

 

 

From: ox...@googlegroups.com <ox...@googlegroups.com> On Behalf Of Marge Belcastro
Sent: 29 September 2025 22:51
To: OxCal <ox...@googlegroups.com>
Subject: Issue with samples when I "plot on curve"

 

You don't often get email from ma...@pdx.edu. Learn why this is important

[EXTERNAL EMAIL]

Hello,

I am trying to wiggle match growth rings to constrain the death of a tree. There are 3 samples that are closely spaced in time (rings 130, 141, and 162) when viewed on “plot dates” things look correct but when I view “plot on curve” I run into an issue where sample 141 is plotting higher than samples 130 and 162 which seem to overlap. 

 

Here is a bit of the coding used for these three samples:

  Gap( 6);

   R_Date( "NB2019_S162", 1825, 20);

   Gap( 21);

   R_Date( "NB2019_S141", 1795, 20);

   Gap( 11);

   R_Date( "Netarts_NS_TR_130", 1820, 20);

   Gap( 90);

 

I have verified that all C14 ages entered are correct and I looked at the agreement indices and the lowest agreement is 72% (sample 141) and samples 162 and 168 are both above 75%. 

 

I am wondering if there is an issue with the curve I am using for calibration (regional vs local).

 

Below is the OxCal table view of the samples we are working with:

 

Any suggestions or advice you can give me would be greatly appreciated.

 

-Marge

 

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[Marge Belcastro]

Hello and thank you for getting back to me.
Below are some photos to hopefully illuminate the issue I am running into.

Here are all of my samples shown on "plot dates"

Everything looks good, but when I use "plot on curve" sample 141 suddenly plots above 130 and 162 (see below)

Below are 2 zoomed in screenshots to help see the problem:

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[Derek Hamilton]

Hi Marge,

There is no actual problem with how OxCal is handling the data. “Plot on curve” does not display the dates with any model-defined sequencing as “Plot dates” does. It simply plots all your dates against the calibration curve and they are ordered based on the measured radiocarbon age (y-axis).

Best wishes,

Derek

On 30 Sep 2025, at 19:16, Marge Belcastro <ma...@pdx.edu> wrote:



Hello and thank you for getting back to me.
Below are some photos to hopefully illuminate the issue I am running into.

Here are all of my samples shown on "plot dates"

<image.png>

Everything looks good, but when I use "plot on curve" sample 141 suddenly plots above 130 and 162 (see below)

<image.png>

Below are 2 zoomed in screenshots to help see the problem:

<image.png>

On Tue, Sep 30, 2025 at 12:23 AM MILLARD, ANDREW R. <a.r.m...@durham.ac.uk> wrote:

Hello Marge,

 

With your code snippet I get this plot on curve, which looks fine to me. Each date is vertically at its mean radiocarbon date and the probability distributions are the same as in plot dates.  What are you seeing?

<image001.png>

 

Best wishes 

Andrew 

-- 

Prof. Andrew Millard 

Department of Archaeology,

Durham University, UK 

Email: A.R.M...@durham.ac.uk  

Personal page: https://www.durham.ac.uk/staff/a-r-millard/ 

Dunbar 1650 MOOC: https://www.futurelearn.com/courses/battle-of-dunbar-1650 

 

<image002.png>

 

From: ox...@googlegroups.com <ox...@googlegroups.com> On Behalf Of Marge Belcastro
Sent: 29 September 2025 22:51
To: OxCal <ox...@googlegroups.com>
Subject: Issue with samples when I "plot on curve"

 

You don't often get email from ma...@pdx.edu. Learn why this is important

[EXTERNAL EMAIL]

Hello,

I am trying to wiggle match growth rings to constrain the death of a tree. There are 3 samples that are closely spaced in time (rings 130, 141, and 162) when viewed on “plot dates” things look correct but when I view “plot on curve” I run into an issue where sample 141 is plotting higher than samples 130 and 162 which seem to overlap. 

 

Here is a bit of the coding used for these three samples:

  Gap( 6);

   R_Date( "NB2019_S162", 1825, 20);

   Gap( 21);

   R_Date( "NB2019_S141", 1795, 20);

   Gap( 11);

   R_Date( "Netarts_NS_TR_130", 1820, 20);

   Gap( 90);

 

I have verified that all C14 ages entered are correct and I looked at the agreement indices and the lowest agreement is 72% (sample 141) and samples 162 and 168 are both above 75%. 

 

I am wondering if there is an issue with the curve I am using for calibration (regional vs local).

 

Below is the OxCal table view of the samples we are working with:

<image003.png>

 

Any suggestions or advice you can give me would be greatly appreciated.

 

-Marge

 

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[Richard Staff]

Hi Marge,

Personally, if I am ever wanting to plot data on the Curve view, I view it without the full distributions plotted.

(Edit > Format > untick Distributions)

For the subset of your model, this would look something like:

(The dark grey boxes show the 68.3% confidence ranges and light grey shows the 95.4% ranges.)

The relative spacing on the x-axis (calibrated time) is defined by the Gaps in your D_Sequence, so you can then just imagine the whole model shifting left or right to find the optimal fit on the calibration curve... (Which OxCal has, of course, done for us.)

Hope that helps!

Richard  😊

To view this discussion visit https://groups.google.com/d/msgid/oxcal/DDEC2F3E-848E-46B2-8277-62BD4FEDEAB1%40mac.com.

[Marge Belcastro]

Thank you!

To view this discussion visit https://groups.google.com/d/msgid/oxcal/LO4P265MB4405A387C14271C70B72ACE5E8E6A%40LO4P265MB4405.GBRP265.PROD.OUTLOOK.COM.

[Marge Belcastro]

Thank you for the clarification.

To view this discussion visit https://groups.google.com/d/msgid/oxcal/DDEC2F3E-848E-46B2-8277-62BD4FEDEAB1%40mac.com.