projections to calculate perimeter (small areas)

[joycemaia]

Hello,

Someone let me know what the best projection to calculate the perimeter of polygons. I need to calculate the perimeter (border) of all countries of the world. They are grouped by continent.

Must also calculate the area of the cities of these countries.

I used the projection Albers for each continent, but realized that for small areas it returns 0.

thank you all

[joycemaia]

Hello Sephe Fox,

I made some changes here and not getting null values.

However, as I said in another post, the calculated perimeter are very disparate with which the CIA informs.

Using Europe as an example:

Sweden

According to the calculation made by the projection of Albers (in my ArcGIS not appear the projection Mollwiede)

Area: 449,918.59 sq km

Perimeter: 25875.88 km

CIA

https://www.cia.gov/library/publications/resources/the-world-factbook/geos/sw.html

Area: 450.295 sq km

Perimeter: costline + Land boundaries (3.218 km + 2.211 km) approx. 5,400km

You know what might be happening?

thank you

[SepheFox]

What method are you using to calculate your area and perimeter?

[SepheFox]

For Sweden, I calculated a perimeter of over 16000 miles, which is equivalent to 25000+ km, so your process for calculating geometry may be wrong.

[joycemaia]

Hello Sephe,

I found the projection, my mistake!

I performed the calculation and found the same values as you.

That they are similar to those found by the projection of Albers.

I do not know, but I think there must be some mistake.

On the site of the CIA to Sweden would like their limits (adding the coastal part and regional part) about 5.400 km of perimeter and not 25,000 as pointed out by the calculation made with porojeções

When compared the values for the Vatican, the perimeter calculated by Mollwiede (3.32 km) equals the exposed CIA (3.4 km)

So I think it is a problem involving the shape of the polygon

[SepheFox · Accepted solution]

Well, Joyce, that depends how you define "problem". I'm quite sure that the calculations we are coming up with are correct for this dataset. This wikipedia page explains the issue very well: List of countries by length of coastline - Wikipedia, the free encyclopedia. Basically, the CIA figures are calculated using data digitized at a very different scale, and there is no way to come up with the same numbers, unless you use the same dataset. You can decide to just use the CIA figures, or calculate your own from the GDAM data. I guess if you want to do further analyses that the CIA hasn't provided, then you'll have to trust the data. It certainly sounds like you're doing it correctly.

As an aside, I'm wondering if typing CIA this many times on an internet forum is putting me on some kind of watchlist!

[joycemaia]

Thank Sephe,
I got it finally actually is not a problem but an a choice!

Wiki information from the <List of countries by length of coastline> were enlightening!


again thank you

[SepheFox]

Also, Mollwiede and Goode Holosine are both in Projected Coordinates-->world.

[ChrisDonohue__GISP]

In response to your question:

I do not know if I'm right, but if a polygon have an overlap with another polygon, the merge function creates a new internal polygon to these two areas. So, if shape of the United States has an overlap with the Canada, the merge function will create a new polygon, as if there were another country that does not exist. Is that right?

Merge will not create a new polygon where there is an overlap.  It will overlap the polygons.

ArcGIS Help 10.1

Chris Donohue, GISP

[ChrisDonohue__GISP]

If you do want new polygons created from any overlaps, one approach would be to use the Union tool.  This will create new polygons from the overlapping extents while maintaining the polygons that don't overlap.  Note that overlapping polygons will have the attributes of all the inputs.

Union (Analysis)

ArcGIS Help 10.1

Chris Donohue, GISP

[SepheFox]

I don't know of a better source of free administrative boundaries for the world than GADM, but it contains very few cities. I made a quick calculation of country perimeters using the Mollwiede projection, and I didn't get any zero values for a calculation in miles. I was also going to try it in Goode Homolosine.