1

The first I have found is to have the lowest value and highest value be displayed like the legend shown below (image source)

Legend 1

The second is to have relative numbers at the top and bottom (image source)

Legend 2

Should I display 1-100 items or less than 100 items? Is one method better than or preferred over the other?

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  • 1
    That question is similar, but not the same. Mine is questioning more what to display as far as the text for the high/low values and not the order the swatches will be in. I've edited the question to better reflect that.
    – BDD
    Feb 23 '15 at 21:19
  • I do see the distinction, but I think the comments on that other question also address this one. There is no hard and fast rule or preference. It's entirely opinion based, and may vary with the map audience, the data being presented, and the intent of the map author in terms of 'spin' (or what story they want to tell). Less than 100 implies any number less whereas 1-100 implies at least one value is 1.
    – Chris W
    Feb 23 '15 at 21:28
  • Related question: gis.stackexchange.com/questions/11145 I guess there is distinction enough that they're not the same question, but as both answers have already stated any answer is going to be primarily opinion based. Had I not already voted to close it as a duplicate (and thus can no longer vote on it), I'd vote to close it on that grounds. It's not a bad question, it just doesn't fit on GIS.SE and might be better suited to chat.
    – Chris W
    Feb 24 '15 at 0:01
  • @ChrisW - I think you're wrong on all counts: (i) This is a different question. (ii) It is valid for GIS SE -- as are most cartographic design questions. (ii) Identifying the limits of a class do not necessarily imply either of the limits are reached in the data -- unless those limits definitely exclude ranges that are necessarily not in the data.
    – Martin F
    Feb 24 '15 at 4:13
  • @martinf I have already addressed that this is a different question. Now that the question has been edited, it is a better fit for GIS.SE - you might look at the edit history and note it used to say "which way is better". That is opinion. "Is there a convention" is fact based and appropriate - and the edit is a better route than closing probably. And yes, I should have said could imply, but the point stands - you cannot know how the reader will interpret your presentation of the data, but you can attempt to influence it. Nor can the reader know the full details of the data being presented.
    – Chris W
    Feb 24 '15 at 4:30
2

The answers to this question will be heavily opinion-based.

The quick answer is: there is no answer, or “it depends”.

The slightly longer answer is: it all depends on the data, and on the people who will be looking at the map. For some audiences your first example might be perfect, and for others your second one might be the perfect one. Then again, if the data is not suitable to be shown in either of those ways then the legend will be just a bunch of nonsense, regardless of the audience.

Personally, I prefer the second one, because personally I get annoyed when I have to look at maps that use decimals, like .7 or .1 when showing percentages; to me these decimals usually do not make any sense, and it will remind me of people showing too many decimals when showing latitude and longitude, thinking that they are providing an extra amount of useful detail, but in the end just turn out to be providing the information that they have little understanding of their data. And yet, I have no idea what this first legend you posted is about. Maybe it does make a lot of sense, and maybe those decimals are terribly important. But then again… wait, I need to stop, there is no point on discussing this. You might see where I am going with this... there is no definitive answer.

A lot of legends like the first one are made by people who have no clear understanding or no knowledge at all of classification schemes. Or, on the other hand, who know very well what they are doing, but use their knowledge for malicious purposes. This is quite dangerous, actually, as we tend to believe most “facts” displayed on maps. But what if the person who made the map knew very well how to deceive the reader? Look at political maps, for instance, or a lot of historical maps. Just like projections, legends and classification methods can be used to get points across that are not “completely true”. There is an entire book dedicated to this, and it is called How to Lie with Maps, by Mark Monmonier. We humans have abused about anything you can think of, why not add maps to that list as well?

Even if the classes might be intelligently distributed, we might run into another problem that is often not thought about: how many classes or colors can we actually distinguish? I cannot cite this, but I remember learning that humans cannot easily distinguish more than seven colors. Basically, reading any classification/legend that displays more than seven colors is getting hard to read, with each additional class turning into an increasingly painful experience for the reader. So, more is not always better. But then again, some maps need more classes than you can or want to imagine, like geologic maps, for instance. As someone who enjoys making maps, I always try my best to stay as far away as possible from geologic maps, as they are just not fun to make. OK, time to go to bed before I keep spreading more opinions.

That being said, the truth is out there, keep searching, it might be hidden in some legend on Fox News.

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In my opinion it depends entirely on the audience and intent of the map. For a scientific audience I would lean toward the true minimum and maximum values of the data set (version 1). For a general audience I would lean towards option 2, as it is more intuitive.

But, I would recommend a different color scheme than that used in the second legend, check out color brewer for some suggestions on divergent color ramps.

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  • Thanks for the feedback. I was thinking along the same lines. Also, these aren't legends that I've created. I just grabbed them off the internet as examples.
    – BDD
    Feb 23 '15 at 21:17

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