topic Re: projections to calculate perimeter (small areas) in Map Projections Questions

projections to calculate perimeter (small areas)

joycemaia — Fri, 15 May 2015 02:36:00 GMT

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

Re: projections to calculate perimeter (small areas)

SepheFox — Fri, 15 May 2015 03:09:38 GMT

But even the smallest country has a perimeter of at least a mile, and cities would have some area greater than zero? What method are you using to calculate?

Re: projections to calculate perimeter (small areas)

SepheFox — Fri, 15 May 2015 03:12:42 GMT

This tool might give you some ideas to try though: Projection Wizard. The basic idea would be to preserve area, since that would give you the most accurate perimeter.

Re: projections to calculate perimeter (small areas)

DanPatterson_Retired — Fri, 15 May 2015 07:19:55 GMT

are you using a geodatabase or something else? are the boundaries of the cities shown on the map? what is the precision of the field and what is the area unit? It is quite possible that this could give rise to values being truncated or rounded to 0. Is there any reason you are trying to mix continental scale data with local data?

Re: projections to calculate perimeter (small areas)

joycemaia — Fri, 15 May 2015 14:22:52 GMT

Hello
Look what I did:
1. I took all the available shapes of GDAM for the different political levels
question: Does anyone know another source for geopolitical shapes. For example, for some countries I have not found the cities of division.

2. I have separated by levels: countries, states / provinces and cities.

3. Again, I have separated by continents (to use their projections).

So I have all cities in one shape to South America, in another shape have states and others have the country ... in the same way to all continents.

To perform the calculations I added two columns in the attribute table. A related area and the other related to the perimeter, and so used the function that calculates the respective variables.

But I do not know if the perimeter this close to real because I got zero values.

From what Albers test is more accurate than Lambert in the calculation of the area. This is because I compared with the values of Worldbank <http://data.worldbank.org/indicator/AG.LND.TOTL.K2>

However, when comparing to the boundary value with the CIA provides values <https://www.cia.gov/library/publications/resources/the-world-factbook/fields/2096.html#133 >> The values I calculated are very different.

Remembering that I still have the problem with small areas q have zero boundary value.

Re: projections to calculate perimeter (small areas)

DanPatterson_Retired — Fri, 15 May 2015 14:42:12 GMT

So when you say you used Albers projected data...did it come projected? or was it in a Geographic Coordinate system (GCS) and you used the Project tool to project it to a new file with the desired coordinate system. Or...worst case...you used the Define Projection tool. The only time you use the latter tool is when you add a layer to arcmap and it blurps forth that it doesn't have a coordinate system and you can't project it until it does...at which point a large number of people define it to what they want it to be and what it should be.

Also questions about the source file, is it a shapefile or a featureclass in a geodatabase? What are the field properties in terms of the number of decimal places it can report

Re: projections to calculate perimeter (small areas)

SepheFox — Fri, 15 May 2015 14:51:25 GMT

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.

Re: projections to calculate perimeter (small areas)

joycemaia — Fri, 15 May 2015 15:09:57 GMT

Hello!

Actually all files are shapes and have the following properties:

Geographic Coordinate System: GCS_WGS_1984

Datum: D_WGS_1984

Prime Meridian Greenwich

Angle Unit: Degree

In ArcToolbox:

1. unify the individual shapes of each country according to the continents. This is done to the country level, state and city.

Data management tools -> General -> Merge -> to unite all shapes

2. Change WGD-1984 projection for Albers or Lambert

Data management tools -> Projection and Transformations -> Project -> the output field coordinate system I choose the projection Albers for the respective continent.

Re: projections to calculate perimeter (small areas)

SepheFox — Fri, 15 May 2015 15:22:33 GMT

Merge is for combining several feature classes. Are you sure you're not using dissolve? That's what I used. Anyway, when I downloaded the GDAM data, I was able to download a geodatabase. After dissolving and reprojecting the data. I added a field, type = double, called perimeter, and then used calculate geometry to calculate perimeter in miles. The smallest country was the vatican, with a perimeter of about 2 miles.

Re: projections to calculate perimeter (small areas)

joycemaia — Fri, 15 May 2015 15:44:19 GMT

I did not understand.

I used to merge together the shapes of each country. And then redesign only once.

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?

My premise is that this does not happen because the shapes and geodabases are the same source.

If I use dissolves me I will not lose internal polygons?

Re: projections to calculate perimeter (small areas)

SepheFox — Fri, 15 May 2015 15:50:53 GMT

I downloaded the data for the whole world as one geodatabase. Perhaps you downloaded each country individually? That would take a while since there are over 250 countries.

Re: projections to calculate perimeter (small areas)

joycemaia — Fri, 15 May 2015 16:40:28 GMT

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

Re: projections to calculate perimeter (small areas)

SepheFox — Fri, 15 May 2015 16:54:35 GMT

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

Re: projections to calculate perimeter (small areas)

SepheFox — Fri, 15 May 2015 16:57:56 GMT

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.

Re: projections to calculate perimeter (small areas)

SepheFox — Fri, 15 May 2015 17:11:14 GMT

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

Re: projections to calculate perimeter (small areas)

ChrisDonohue__GISP — Fri, 15 May 2015 17:13:08 GMT

In response to your question:

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

ArcGIS Help 10.1

Chris Donohue, GISP

Re: projections to calculate perimeter (small areas)

ChrisDonohue__GISP — Fri, 15 May 2015 17:16:04 GMT

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

Re: projections to calculate perimeter (small areas)

joycemaia — Sat, 16 May 2015 04:44:25 GMT

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

Re: projections to calculate perimeter (small areas)

curtvprice — Sat, 16 May 2015 04:51:28 GMT

ArcGIS now supports area calculations in geodesic, i.e. referenced to the globe. Seems to me this would be a good approach for your application.

The Calculate Field tool supports this syntax to do this on the fly with GCS (decimal degree) data:

!shape.geodesicArea@squarekilometers!

Calculate Field examples—Help | ArcGIS for Desktop

Re: projections to calculate perimeter (small areas)

SepheFox — Sat, 16 May 2015 05:11:49 GMT

I tried that, and it said disabled.